[MRG] Bregman backend

[ncassereau]

Types of changes

• Docs change / refactoring / dependency upgrade
• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)

Motivation and context / Related issue

Bregman file now fully uses the backend feature.

How has this been tested (if it applies)

Tested on Jean Zay (Linux) with all 3 backends

Checklist

• The documentation is up-to-date with the changes I made.
• I have read the CONTRIBUTING document.
• All tests passed, and additional code has been covered with new tests.

[codecov]

Codecov Report

Merging #280 (0ad21b1) into master (7af8c21) will increase coverage by 0.18%.
The diff coverage is 94.14%.

@@            Coverage Diff             @@
##           master     #280      +/-   ##
==========================================
+ Coverage   92.79%   92.98%   +0.18%     
==========================================
  Files          20       20              
  Lines        3944     4122     +178     
==========================================
+ Hits         3660     3833     +173     
- Misses        284      289       +5     

[rflamary]

that's good a few more comments and we can merge.

Thank you for the wonderful work on the documentation.