[MRG] Domain adaptation and unbalanced solvers with backend support

[ncassereau]

Types of changes

Introducing backend support for da.py

• Backend version of da.py
• Backend version of unbalanced.py (required by an Unbalanced transport class in da.py)
• Backend tests for unbalanced.py
• Backend tests for da.py
• Add a warning for integers cast (Resolves POT generating incorrect result for very simple OT problem #345)
• Solve scipy warning (about scalar search armijo location)

Motivation and context / Related issue

GPU support and Pytorch and automatic differentation, ...
Note that it does not support jax and tensorflow yet because those two backends do not allow item assignment.

How has this been tested (if it applies)

PR checklist

• I have read the CONTRIBUTING document.
• The documentation is up-to-date with the changes I made (check build artifacts).
• All tests passed, and additional code has been covered with new tests.
• I have added the PR and Issue fix to the RELEASES.md file.

[codecov]

Codecov Report

Merging #343 (7039a39) into master (9b9d222) will increase coverage by 0.08%.
The diff coverage is 96.36%.

@@            Coverage Diff             @@
##           master     #343      +/-   ##
==========================================
+ Coverage   93.63%   93.72%   +0.08%     
==========================================
  Files          22       22              
  Lines        5595     5753     +158     
==========================================
+ Hits         5239     5392     +153     
- Misses        356      361       +5     

[rflamary]

This is an impressive PR good work @ncassereau-idris

I'm still a bit hesitant about the type output that ot.emd should return but in the meantime here are a few comments