[MRG] Bregman backend (#280)

* Bregman

* Resolve conflicts

* Bug solve

* Bregman updated for JAX compatibility

* Tests coherence between backend improved

* No longer enforcing 64 bits operations on Jax except for tests

* Now using mixtures, to make backend dependent tests with less code

* Better test skipping code

* Pep8 + test optimizations

* redundancy removed

* Docs

* Typo corrected

* Typo

* Typo

* Docs

* Docs

* pep8

* Backend docs

* Prettier docs

* Mistake corrected

* small changes

* Better wording

Co-authored-by: Rémi Flamary <remi.flamary@gmail.com>

Author: ncassereau

docs/source/all.rst

@@ -14,6 +14,7 @@ API and modules
    :template: module.rst
 
    lp
+   backend
    bregman
    smooth
    gromov