Allow setting of matplotlib backend and layout engine

[taylorbell57]

This PR adds support for inputting matplotlib backend to lib.plots.set_rc().

Additionally, this PR ensures that the matplotlib layout engine is always set, which resolves #647

I've tested this on my computer and importing eureka is enough to make sure all the plots are made successfully, but users should still run the eureka.lib.plots.set_rc(style='eureka') step. I didn't want to force the eureka style on everyone who ever imports eureka, but I needed to make sure the layout engine and figure filetype were defined, so I tried to strike a minimally intrusive balance.

[codecov[bot]]

Codecov Report

Attention: Patch coverage is 64.28571% with 5 lines in your changes missing coverage. Please review.

Project coverage is 54.27%. Comparing base (a939236) to head (e13467d).

Files	Patch %	Lines
src/eureka/lib/plots.py	72.72%	3 Missing :warning:
...eureka/S1_detector_processing/update_saturation.py	0.00%	1 Missing :warning:
src/eureka/S6_planet_spectra/s6_spectra.py	0.00%	1 Missing :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #665 +/- ## ========================================== + Coverage 54.24% 54.27% +0.03% ========================================== Files 101 101 Lines 12793 12802 +9 ========================================== + Hits 6939 6948 +9 Misses 5854 5854 ```

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