dfsp-spirit/fsbrain: Experimental DTI track visualization support

This release was needed for technical reasons related to CRAN. Most important changes:

• Support for visualizing TRK and TCK files (Fiber track files from Diffusion Tensor Imaging) created in Diffusion Toolkit and MRtrix3
• Delete all unit test data after tests have been run if on CRAN

The DTI visualization support is still a bit experimental, and the visualization style may change in the future.