BF: Don't assume volumes with a b-value greater than 100 are diffusion-weighted

In !14 (merged), eddy_quad was modified to use the --b_range value passed to eddy as its threshold for distinguishing between b0 and diffusion-weighted volumes. However there were still a few instances where eddy_quad was using a hard-coded value of 100. This should now be fixed.

This MR also makes some changes so that eddy_[s]quad do not assume that all NIfTI files are stored in .nii.gz format.

Closes #2 (closed)