Fix DA cost correction when cost limit is set to Inf

[kachayev]

Types of changes

Cost correction for mismatched labels in ot.da.BaseTransport fit is fixed. The original change was introduced in the #587.

Motivation and context / Related issue

Previous version of the procedure was failing when limit_max is set to Inf, resulting in self.cost_ having NANs.

How has this been tested (if it applies)

Additional checks in the DA tests to ensure that cost does not contain NANs.

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

Merging #593 (72f1bac) into master (98a58d2) will increase coverage by 0.00%.
The diff coverage is 100.00%.

Additional details and impacted files

@@ Coverage Diff @@## master #593 +/- ##
=======================================
Coverage 96.75% 96.75% =======================================
Files 77 77 Lines 15939 15961 +22 =======================================
+ Hits 15421 15443 +22 
Misses 518 518 

[rflamary]

LGTM