Add save to xspec varenergy

[matteobachetti]

This adds the "save to xspec" functionality to variability-energy spectra.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 96.17%. Comparing base (7724c74) to head (04a5840).
⚠️ Report is 14 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #963   +/-   ##
=======================================
  Coverage   96.16%   96.17%           
=======================================
  Files          48       48           
  Lines        9937     9959   +22     
=======================================
+ Hits         9556     9578   +22     
  Misses        381      381           

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[matteolucchini1]

Other than the checks not passing, this -mostly- looks good to me.

The only comment I have is that we should be incredibly careful with generating products (like time-averaged spectra) to files as if they were identical to what pipelines produced - we've had this discussion before where we have discrepancies between e.g. what nicerl3 produces, and what we get when we just build energy resolved lightcurves, spectra etc with Stingray going straight from the event files.

My recommendation is to at least raise a warning for users, and to tell them to check that whatever they are building with stingray is consistent with the other mission pipelines.

[matteobachetti]

@matteolucchini1 very good point, I'll extend the existing warning for missing keywords along those lines.

[matteolucchini1]

Alright LGTM now, just need to get it to pass all the tests :)

[matteobachetti]

@matteolucchini1 it was a temporary glitch due to an issue with the Contribution covenant website. It works now.