Rename sh2power to sh2metric, new "entropy" operation

[Lestropie]

Had previously thought about this from the perspective of registration, but have also been looking at a collaborator dataset where doing satisfactory masking for FBA has been difficult, and using that to revised FBA documentation (#2685).

Here is the WM FOD l=0 image, which you can see doesn't do a very good job of separating what one would like to analyse vs. not:

Then this is the entropy of the WM FOD template (normalised & inverted):

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• [ ] Implement the Spherical Harmonic Entropy (SHE) metric as per: https://arxiv.org/pdf/1805.08084 This may yield the same or similar result much faster

• [ ] Add command-line option to modify the direction set that's used for amplitude sampling

• [ ] Implement entropy operations for:

  • [ ] mrmath
  • [ ] fixel2voxel

• [ ] Revisit templated entropy calculation code For some reason had trouble specifying the logarithm function as a template. The switch statement should get optimised out by the compiler, but maybe I'll have another go before merging.

[Lestropie]

Was potentially a little too idealistic about this metric being useful as a scalar surrogate for multi-contrast registration (as multiple SH images can be provided as input to the entropy calculation). Kind of interesting, and maybe useful in specific use cases such as the unusual template above, but not a game-changer.

Single-subject HCP data: