Types of changes

• Implement CG solvers for partial FGW
• Add to solve_gromov

Motivation and context / Related issue

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 Report

Attention: Patch coverage is 99.08257% with 2 lines in your changes missing coverage. Please review.

Project coverage is 97.05%. Comparing base (4435898) to head (4bbc37c).
Report is 1 commits behind head on master.

@@            Coverage Diff             @@
##           master     #687      +/-   ##
==========================================
+ Coverage   97.03%   97.05%   +0.02%     
==========================================
  Files          98       98              
  Lines       19639    19854     +215     
==========================================
+ Hits        19056    19269     +213     
- Misses        583      585       +2