FT_SOURCEPARCELLATE combines the source-reconstruction parameters over the parcels, for
 example by averaging all the values in the anatomically or functionally labeled parcel.

 Use as
    output = ft_sourceparcellate(cfg, source, parcellation)
 where the input source is a 2D surface-based or 3-D voxel-based source grid that was for
 example obtained from FT_SOURCEANALYSIS or FT_COMPUTE_LEADFIELD. The input parcellation is
 described in detail in FT_DATATYPE_PARCELLATION (2-D) or FT_DATATYPE_SEGMENTATION (3-D) and
 can be obtained from FT_READ_ATLAS or from a custom parcellation/segmentation for your
 individual subject. The output is a channel-based representation with the combined (e.g.
 averaged) representation of the source parameters per parcel.

 The configuration "cfg" is a structure that can contain the following fields
   cfg.method       = string, method to combine the values, see below (default = 'mean')
   cfg.parcellation = string, fieldname that contains the desired parcellation
   cfg.parameter    = cell-array with strings, fields that should be parcellated (default = 'all')

 The values within a parcel or parcel-combination can be combined with different methods:
   'mean'      compute the mean
   'median'    compute the median (unsupported for fields that are represented in a cell-array)
   'eig'       compute the largest eigenvector
   'min'       take the minimal value
   'max'       take the maximal value
   'maxabs'    take the signed maxabs value