Data validation commands

[Lestropie]

Explicit Issue listing following discussion in #2063.

• [ ] Implement functions for thorough checking of validity of various representations of data:
  • [ ] 5TT (already present in 5ttcheck; move to back-end) (documentation)
  • [ ] dir (azimuth-elevation pairs / XYZ triplets / diffusion gradient tables)
  • [ ] fixel (fixel directory format)
  • [ ] label (hard segmentations; support for soft segmentations in #2376)
  • [ ] mesh (surface data; potentially also scalars expressed on surface)
  • [ ] peaks (4D series of XYZ triplets)
  • [ ] tck (streamlines data; see also -count in tckstats)
  • [ ] tsf (track scalar file; see also tsfvalidate)
  • [ ] warp (non-linear warp field; displacement, deformation, or "full")
  • Others?
• [ ] For all data representations, implement command-line binary "*validate" for explicit manual checking of input data
• [ ] Additionally execute checking function(s) relevant for executed command if -debug is specified at runtime

[Lestropie]

@jdtournier Would this be a good one for @daljit46 to gain some basic familiarity with the software and types of data we deal with?