Converter for FreeSurfer format `mgh`

[zhao-cy]

Some differences from other converters:

• needs to combine lh and rh data when --> hdf5; needs to separate lh and rh when --> .mgh
• source filename (in column names of hdf5 file): something like lrh, instead of just lh or rh

Testing against: FreeSurfer has in-house qdec (link here) which does statistics; can compare this converter + ModelArray results versus that one.

[zhao-cy]

See example scripts from Steven in issue #14
Currently what he does are:

• FS output (e.g., lh.thickness*.mgh and rh.thickness*.mgh) --> harmonize --> convert np.array into HDF5 for ModelArray, with lh and rh combined [in python]
• ModelArray output dataframe --> directly saves as FS format with lh and rh separated [in R]

[zhao-cy]

Finally, probably also consider adding group-level mask and individual masks as inputs, just like those for voxel-wise data's converter ConVoxel.