Eddy correction fixes for high b-value shelled datasets

Added:

• None

Changed

• Replaced preprocessing.util.bvec_is_fullsphere() and
  preprocessing.util.vecs_are_fullsphere() with
  preprocessing.mrinfoutil.is_fullsphere(). Even though datasets
  may be half-shelled, it is inaccurate to label them as such because
  distortion relative to b-value is not linear. As such, the
  slm=linear makes no sense. This new method performs the proper
  checks required before labelling a DWI as fully-shelled. A DWI is
  half-shelled iff max B-value is less than 3000 AND the norm of the
  mean direction vector is more than 0.3.

Removed

• See above