[MRG] Fiw proximal point algorithm

[thibaut-germain]

Types of changes

Fixing sign issue in proximal_bregman_log_plan_batch changing C + inner_reg * log(T) to C - inner_reg * log(T).

Motivation and context / Related issue

Issue #842

How has this been tested (if it applies)

PR checklist

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

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 96.86%. Comparing base (bb58c31) to head (cd8cd36).

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #844   +/-   ##
=======================================
  Coverage   96.86%   96.86%           
=======================================
  Files         128      128           
  Lines       25938    25947    +9     
=======================================
+ Hits        25124    25133    +9     
  Misses        814      814           

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