FT_MEGPLANAR computes planar MEG gradients gradients for raw data or average
 event-related field data. It can also convert frequency-domain data that was computed
 using FT_FREQANALYSIS, as long as it contains the complex-valued fourierspcrm and not
 only the powspctrm.

 Use as
    [interp] = ft_megplanar(cfg, data)
 where the input data corresponds to the output from FT_PREPROCESSING,
 FT_TIMELOCKANALYSIS or FT_FREQANALYSIS (with output='fourierspcrm').

 The configuration should contain
   cfg.planarmethod   = string, can be 'sincos', 'orig', 'fitplane', 'sourceproject' (default = 'sincos')
   cfg.channel        =  Nx1 cell-array with selection of channels (default = 'MEG'), see FT_CHANNELSELECTION for details
   cfg.trials         = 'all' or a selection given as a 1xN vector (default = 'all')

 The methods orig, sincos and fitplane are all based on a neighbourhood interpolation.
 For these methods you need to specify
   cfg.neighbours     = neighbourhood structure, see FT_PREPARE_NEIGHBOURS

 In the 'sourceproject' method a minumum current estimate is done using a large number
 of dipoles that are placed in the upper layer of the brain surface, followed by a
 forward computation towards a planar gradiometer array. This requires the
 specification of a volume conduction model of the head and of a source model. The
 'sourceproject' method is not supported for frequency domain data.

 A dipole layer representing the brain surface must be specified with
   cfg.inwardshift = depth of the source layer relative to the head model surface (default = 2.5 cm, which is appropriate for a skin-based head model)
   cfg.spheremesh  = number of dipoles in the source layer (default = 642)
   cfg.pruneratio  = for singular values, default is 1e-3
   cfg.headshape   = a filename containing headshape, a structure containing a
                     single triangulated boundary, or a Nx3 matrix with surface
 If no headshape is specified, the dipole layer will be based on the inner compartment
 of the volume conduction model.

 The volume conduction model of the head should be specified as
   cfg.headmodel     = structure with volume conduction model, see FT_PREPARE_HEADMODEL

 The following cfg fields are optional:

 To facilitate data-handling and distributed computing you can use
   cfg.inputfile   =  ...
   cfg.outputfile  =  ...
 If you specify one of these (or both) the input data will be read from a *.mat
 file on disk and/or the output data will be written to a *.mat file. These mat
 files should contain only a single variable, corresponding with the
 input/output structure.