FT_MEGREALIGN interpolates MEG data towards standard gradiometer locations by
 projecting the individual timelocked data towards a coarse source reconstructed
 representation and computing the magnetic field on the standard gradiometer
 locations.

 Use as
   [interp] = ft_megrealign(cfg, data)

 Required configuration options:
   cfg.template
   cfg.inwardshift

 The new gradiometer definition is obtained from a template dataset,
 or can be constructed by averaging the gradiometer positions over
 multiple datasets.
   cfg.template       = single dataset that serves as template
   cfg.template(1..N) = datasets that are averaged into the standard

 The realignment is done by computing a minumum norm estimate using a
 large number of dipoles that are placed in the upper layer of the brain
 surface, followed by a forward computation towards the template
 gradiometer array. This requires the specification of a volume conduction
 model of the head and of a source model.

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

 A source model (i.e. a superficial layer with distributed sources) can be
 constructed from a headshape file, or from inner surface of the volume conduction
 model using FT_PREPARE_SOIURCEMODEL using the following options
   cfg.spheremesh  = number of dipoles in the source layer (default = 642)
   cfg.inwardshift = depth of the source layer relative to the headshape
                     surface or volume conduction model (no default
                     supplied, see below)
   cfg.headshape   = a filename containing headshape, a structure containing a
                     single triangulated boundary, or a Nx3 matrix with surface
                     points

 If you specify a headshape and it describes the skin surface, you should specify an
 inward shift of 2.5 cm.

 For a single-sphere or a local-spheres volume conduction model based on the skin
 surface, an inward shift of 2.5 cm is reasonable.

 For a single-sphere or a local-spheres volume conduction model based on the brain
 surface, you should probably use an inward shift of about 1 cm.

 For a realistic single-shell volume conduction model based on the brain surface, you
 should probably use an inward shift of about 1 cm.

 Other options are
 cfg.pruneratio  = for singular values, default is 1e-3
 cfg.verify      = 'yes' or 'no', show the percentage difference (default = 'yes')
 cfg.feedback    = 'yes' or 'no' (default = 'no')
 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')

 This implements the method described by T.R. Knosche, Transformation
 of whole-head MEG recordings between different sensor positions.
 Biomed Tech (Berl). 2002 Mar;47(3):59-62. For more information and
 related methods, see Stolk et al., Online and offline tools for head
 movement compensation in MEG. NeuroImage, 2012.

 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.

 See also FT_PREPARE_LOCALSPHERES, FT_PREPARE_SINGLESHELL