Types of changes

Split ot.bregman.py within a sub-repository ot/bregman/ containing files:

• ot.bregman.__init__.py
• ._utils.py : maybe not necessary. For now it is used in _barycenter.py and _dictionary.py. But it might be good for future integration, plus there are a lot of recurrent operations in this repository so it might be useful to factor it in the future.
• ._barycenter.py
• ._convolutional.py
• ._empirical
• ._screenkhorn
• ._dictionary

Motivation and context / Related issue

Address request in Issue #567

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.