dfsp-spirit/freesurferformats: DTI track format support: TCK and TRK

This release adds support for 2 different Diffusion Tensor Imaging file formats:

• the MRtrix TCK format (in function read.dti.tck) and
• the trk format used by the Diffusion Toolkit and TrackVis in (function read.dti.trk).

I would have waited longer, but a release was needed for technical reasons related to CRAN.