Implementation of Quantum Decoherence in vacuum

[santosmv]

The effects are combined with the MSW effect. In this case, the flavor conversion from quantum decoherence happens in vacuum (as in the MSC^e of arXiv:2306.17591), and no Earth matter effects are included in this version.

[mcolomermolla]

I could use the QD class without problems in a FlavorTransformation notebook to build the flux and spectrum, which follows the expected trend from the paper. I did not find any issue with the flavor transition probabilities implementation, it follows the same scheme as for the other (i.e. AdiabaticSW). To me, this PR is good to go.

[JostMigenda]

Sorry for the delay! I just ran the unit tests (GitHub doesn't run them automatically for PRs from first-time contributors, I think) and unfortunately, these are currently failing; it looks like some issue with units. @santosmv, can you please take a look at this?

[santosmv]

Sure, I'll take a look at this.

[santosmv]

It was just missing units of quantum decoherence parameters in the test_04_xforms.py. Upon running this test, some issues were raised in the assertEqual methods, which were also fixed.

[JostMigenda]

Just added a minor edit to clean up some unnecessary unit conversions. (Usually, AstroPy handles those automatically as needed; manual conversions are mainly needed at the end to format for output.)

Otherwise, @mcolomermolla already posted an approving review above; so now that the test succeed, I’ll go ahead and merge. Many thanks @santosmv for your work and congratulations on your first SNEWPY PR! 🎉