FT_ELECTRODEPLACEMENT allows manual placement of electrodes on a MRI scan, CT scan
 or on a triangulated surface of the head. This function supports different methods.

 VOLUME - Navigate an orthographic display of a volume (e.g. CT or MRI scan), and
 assign an electrode label to the current crosshair location by clicking on a label
 in the eletrode list. You can undo the selection by clicking on the same label
 again. The electrode labels shown in the list can be prespecified using cfg.channel
 when calling ft_electrodeplacement. The zoom slider allows zoomi/ng in at the
 location of the crosshair. The intensity sliders allow thresholding the image's low
 and high values. The magnet feature transports the crosshair to the nearest peak
 intensity voxel, within a certain voxel radius of the selected location. The labels
 feature displays the labels of the selected electrodes within the orthoplot. The
 global feature allows toggling the view between all and near-crosshair
 markers. The scan feature allows toggling between scans when another scan
 is given as input.

 HEADSHAPE - Navigate a triangulated scalp (for EEG) or brain (for ECoG) surface,
 and assign an electrode location by clicking on the surface. The electrode is
 placed on the triangulation itself.

 1020 - Starting from a triangulated scalp surface and the nasion, inion, left and
 right pre-auricular points, this automatically constructs and follows contours over
 the surface according to the 5% system. Electrodes are placed at certain relative
 distances along these countours. This is an extension of the 10-20 standard
 electrode placement system and includes the 20%, 10% and 5% locations. See
 "Oostenveld R, Praamstra P. The five percent electrode system for high-resolution
 EEG and ERP measurements. Clin Neurophysiol. 2001 Apr;112(4):713-9" for details.

 SHAFT - This is for placing electrodes along a linear sEEG shaft. The tip of the
 shaft, another point along the shaft and the distance between the electrodes should
 be specified. If the shaft is not straight but curved, you should specify multiple
 (at least two) points along the saft, i.e. specify cfg.shaft.along=Nx3 for N
 points. The number of electrodes that is placed is determined from cfg.channel.

 GRID - This is for placing electrodes on a regular MxN ECoG grid. Each of the four
 cornerpoints of the grid must be specified, along with the dimensions of the grid.
 Following piecewise linear placement of the electrodes on the grid, you can use
 FT_ELECTRODEREALIGN with cfg.method='project' to project them to the curved brain
 surface.

 Use as
   [elec] = ft_electrodeplacement(cfg, ct)
   [elec] = ft_electrodeplacement(cfg, ct, mri, ..)
 where the input mri should be an anatomical CT or MRI volume, or
   [elec] = ft_electrodeplacement(cfg, headshape)
 where the input headshape should be a surface triangulation.

 The configuration can contain the following options
   cfg.method         = string representing the method for placing the electrodes
                        'volume'          interactively locate electrodes on three orthogonal slices of a volumetric MRI or CT scan
                        'headshape'       interactively locate electrodes on a head surface
                        '1020'            automatically locate electrodes on a head surface according to the 10-20 system
                        'shaft'           automatically locate electrodes along a linear sEEG shaft
                        'grid'            automatically locate electrodes on a MxN ECoG grid

 The following options apply to the 'volume' method
   cfg.parameter      = string, field in data (default = 'anatomy' if present in data)
   cfg.channel        = Nx1 cell-array with selection of channels (default = {'1' '2' ...})
   cfg.elec           = struct containing previously placed electrodes (this overwrites cfg.channel)
   cfg.clim           = color range of the data (default = [0 1], i.e. the full range)
   cfg.magtype        = string representing the 'magnet' type used for placing the electrodes
                        'peakweighted'    place electrodes at weighted peak intensity voxel (default)
                        'troughweighted'  place electrodes at weighted trough intensity voxel
                        'peak'            place electrodes at peak intensity voxel (default)
                        'trough'          place electrodes at trough intensity voxel
                        'weighted'        place electrodes at center-of-mass
   cfg.magradius      = number representing the radius for the cfg.magtype based search (default = 3)

 The following options apply to the '1020' method
   cfg.fiducial.nas   = 1x3 vector with coordinates
   cfg.fiducial.ini   = 1x3 vector with coordinates
   cfg.fiducial.lpa   = 1x3 vector with coordinates
   cfg.fiducial.rpa   = 1x3 vector with coordinates
   cfg.feedback       = string, can be 'yes' or 'no' for detailled feedback (default = 'yes')

 The following options apply to the 'shaft' method
   cfg.shaft.tip      = 1x3 position of the electrode at the tip of the shaft
   cfg.shaft.along    = 1x3 or Nx3 positions along the shaft
   cfg.shaft.distance = scalar, distance between electrodes

 The following options apply to the 'grid' method
   cfg.grid.corner1   = 1x3 position of the upper left corner point
   cfg.grid.corner2   = 1x3 position of the upper right corner point
   cfg.grid.corner3   = 1x3 position of the lower left corner point
   cfg.grid.corner4   = 1x3 position of the lower right corner point

 In the interactive 'headshape' and 'volume' methods you can click once on an
 already assigned electrode to jump to that electrode position and you can click
 twice to remove the assigned electrode position.

 See also FT_ELECTRODEREALIGN, FT_VOLUMEREALIGN, FT_VOLUMESEGMENT, FT_PREPARE_MESH