FT_SOURCEINTERPOLATE interpolates source activity or statistical maps onto the
 voxels or vertices of an anatomical description of the brain.  Both the functional
 and the anatomical data can either describe a volumetric 3D regular grid, a
 triangulated description of the cortical sheet or an arbitrary cloud of points.

 The functional data in the output data will be interpolated at the locations at
 which the anatomical data are defined. For example, if the anatomical data was
 volumetric, the output data is a volume-structure, containing the resliced source
 and the anatomical volume that can be visualized using FT_SOURCEPLOT or written to
 file using FT_SOURCEWRITE.

 The following scenarios are possible:

 - Both functional data and anatomical data are defined on 3D regular grids, for
   example with a low-res grid for the functional data and a high-res grid for the
   anatomy.

 - The functional data is defined on a 3D regular grid of source positions
   and the anatomical data is defined on an irregular point cloud, which can be a
   2D triangulated mesh.

 - The functional data is defined on an irregular point cloud, which can be a 2D
   triangulated mesh, and the anatomical data is defined on a 3D regular grid.

 - Both the functional and the anatomical data are defined on an irregular
   point cloud, which can be a 2D triangulated mesh.

 - The functional data is defined on a low resolution 2D triangulated mesh and the
   anatomical data is defined on a high resolution mesh, where the low-res vertices
   form a subset of the high-res vertices. This allows for mesh based interpolation.
   The algorithm currently implemented is so-called 'smudging' as it is also applied
   by the MNE-suite software.

 Use as
   [interp] = ft_sourceinterpolate(cfg, source, anatomy)
   [interp] = ft_sourceinterpolate(cfg, stat,   anatomy)
 where
   source  is the output of FT_SOURCEANALYSIS
   stat    is the output of FT_SOURCESTATISTICS
   anatomy is the output of FT_READ_MRI or one of the FT_VOLUMExxx functions,
           a cortical sheet that was read with FT_READ_HEADSHAPE, or a regular
           3D grid created with FT_PREPARE_SOURCEMODEL.
 and cfg is a structure with any of the following fields
   cfg.parameter     = string (or cell-array) of the parameter(s) to be interpolated
   cfg.downsample    = integer number (default = 1, i.e. no downsampling)
   cfg.interpmethod  = string, can be 'nearest', 'linear', 'cubic',  'spline', 'sphere_avg' or 'smudge' (default = 'linear for interpolating two 3D volumes, 'nearest' for all other cases)

 The supported interpolation methods are 'nearest', 'linear', 'cubic' or 'spline'
 for interpolating two 3D volumes onto each other. For all other cases the supported
 interpolation methods are 'nearest', 'sphere_avg' or 'smudge'.

 The functional and anatomical data should be expressed in the same
 coordinate sytem, i.e. either both in MEG headcoordinates (NAS/LPA/RPA)
 or both in SPM coordinates (AC/PC).

 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_READ_MRI, FT_READ_HEADSHAPE, FT_SOURCEPLOT, FT_SOURCEANALYSIS,
 FT_SOURCEWRITE