[MRG] ot.lp reorganizing

[eloitanguy]

Types of changes

• create ot.lp.network_flow.py to store ot.emd and related functions which used to be in ot/lp/__init__.py
• create ot.lp.barycenter_solvers.py to store free_support_barycenter functions

Motivation and context / Related issue

How has this been tested (if it applies)

• full coverage in test/test_ot.py

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

Attention: Patch coverage is 93.70370% with 17 lines in your changes missing coverage. Please review.

Project coverage is 97.08%. Comparing base (200322b) to head (26748eb).
Report is 1 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #714   +/-   ##
=======================================
  Coverage   97.08%   97.08%           
=======================================
  Files          98      100    +2     
  Lines       19937    19947   +10     
=======================================
+ Hits        19355    19365   +10     
  Misses        582      582