mrtrix easyblock should also copy share to installdir

[HPC-UniOldenburg]

one of our users noted that the command 5ttgen hsvs <input> <output> errors in MRtrix/3.0.3 with

[ERROR] Required input file missing (expected location: %(installdir)s/share/mrtrix3/_5ttgen/hsvs/HippSubfields.txt

I guess this affects all versions of MRtrix >=3.0. Probably a fix in the easyblock mrtrix.py (adding after line 86) is the best solution, although it could also be added as a postinstallcmd in the easyconfigs.