fod2metric: Change to fod2fixel and fixel2voxel

Generating scalar images based on FODs is now broken into two steps:
- The FOD segmentation is handled in fod2fixel (renamed from fod2metric to better reflect the operation being performed), which now only writes fixel output images.
- fixel2voxel now has a much wider range of operations it can apply for collapsing fixel data down to a scalar image.
Also added an easy option for disabling all segmentation thresholds, so that all identified FOD lobes are retained.
Closes #129.