fix: check for NaNs in emd loss matrix

[bobluppes]

Types of changes

This PR introduces an additional check for NaNs in the loss matrix of the emd computation. If NaNs are detected we raise an error in order to protect against segfaults in the C++ backend.

Motivation and context / Related issue

The motivation of this PR is to fail more gracefully in cases of NaN costs. Closes #469

How has this been tested (if it applies)

Added new tests.

PR checklist

• [ ] 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.
• [ ] I have added the PR and Issue fix to the RELEASES.md file.

⚠️ Some notes on the checklist above:

• I did not find a CONTRIBUTING.md
• While I ran all related tests, I did not run the entire suite (due to some modules missing on my system). I assume the entire suite is run in the CI?
• Please let me know if this is something you would like a release issue for. If so please let me know.