Discussion: Addition of figure tests using pytest-mpl

[devanshshukla99]

Stingray's test suite produces a lot of plots, however, there are no tests comparing/verifying the plots to a previous version/hash.
This is supposed to be mitigated by other tests, but there can be issues which might manifest themselves into the end product itself (figures). Hence, the idea of verifying the plots to a previous binary/hash.

This can be solved by using pytest-mpl via either comparing the figure themselves or the figure hashes (preferred), as followed by astropy, sunpy, + and it's worth discussing if we want to implement a similar one in stingray, which I'll be happy to do.

The implementation will most likely be heavily inspired by astropy's figure_test, which uses a decorator and a JSON hash file inside the package for comparison.

Secondly, sunpy has a separate repo for storing ref binary files, which we can play it by the ear and implement if and when required.