Xspec gain

[KriSun95]

PR Description

As described in #147 , this draft shows how the gain can be applied in sunkit-spex as it is applied in XSPEC. More discussion is needed but this should be somewhere to comment on.

We change the gain correction being applied to edit the photon axis grid of the SRM before the total count model calculation as opposed to the current behaviour where the gain is applied after the photon model is folded through the response.

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 44.73684% with 21 lines in your changes missing coverage. Please review.

Please upload report for BASE (main@eb860e9). Learn more about missing BASE report.

Files	Patch %	Lines
sunkit_spex/fitting_legacy/fitter.py	44.73%	21 Missing :warning:

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #148   +/-   ##
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  Coverage        ?   55.66%           
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  Files           ?       20           
  Lines           ?     3196           
  Branches        ?        0           
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  Hits            ?     1779           
  Misses          ?     1417           
  Partials        ?        0           

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