WIP: Single-shell FWDTI

[arokem]

Supersedes #2699, because of the complexity of the rebase.

[codecov]

Codecov Report

Merging #2705 (1144e3b) into master (5394ac5) will decrease coverage by 7.50%.
The diff coverage is 72.63%.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #2705      +/-   ##
==========================================
- Coverage   91.38%   83.89%   -7.50%     
==========================================
  Files         254      133     -121     
  Lines       33851    18662   -15189     
  Branches     3569     3053     -516     
==========================================
- Hits        30936    15656   -15280     
- Misses       2111     2258     +147     
+ Partials      804      748      -56     

Impacted Files	Coverage Δ
dipy/align/__init__.py	100.00% <ø> (ø)
dipy/boots/resampling.py	20.77% <ø> (ø)
dipy/core/subdivide_octahedron.py	75.00% <ø> (ø)
dipy/denoise/nlmeans.py	100.00% <ø> (ø)
dipy/denoise/non_local_means.py	100.00% <ø> (ø)
dipy/io/dpy.py	100.00% <ø> (+1.78%)	⬆️
dipy/io/pickles.py	100.00% <ø> (ø)
dipy/io/vtk.py	85.71% <ø> (-3.58%)	⬇️
dipy/pkg_info.py	25.92% <ø> (ø)
dipy/reconst/base.py	88.88% <ø> (ø)
... and 93 more

... and 138 files with indirect coverage changes