Investigate source code mutation testing

[SamWilsn]

[SamWilsn]

We should include the patches against EELS.

• The forced fail test is fine to just include.
• Instead of skipping problematic mutant tests, can we tag them instead and exclude them with a -m 'not angry-mutant' or some such? Same idea as the slow tests.

[SamWilsn]

@souradeep-das has most (if not all) of this working now 🎉

I'm going to leave this open until we get it running somewhere.

[github-actions]

This issue has had no activity for 60 days and has been marked as stale.