Fits event lists

[matteobachetti]

Introduces a new FITS reader class, that lazy loads the data until a slice is required (in which case, the wanted StingrayTimeseries object is created).

[codecov]

Codecov Report

Attention: Patch coverage is 92.62821% with 23 lines in your changes missing coverage. Please review.

Project coverage is 79.52%. Comparing base (aee67bb) to head (0f4a16b).
Report is 6 commits behind head on main.

Files with missing lines	Patch %	Lines
stingray/io.py	91.24%	19 Missing ⚠️
stingray/events.py	55.55%	4 Missing ⚠️

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HEAD has 13 uploads less than BASE

Flag	BASE (aee67bb)	HEAD (0f4a16b)
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Additional details and impacted files

@@             Coverage Diff             @@
##             main     #834       +/-   ##
===========================================
- Coverage   96.11%   79.52%   -16.59%     
===========================================
  Files          48       48               
  Lines        9371     9667      +296     
===========================================
- Hits         9007     7688     -1319     
- Misses        364     1979     +1615     

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[matteolucchini1]

Other than the obvious requiring docstrings, I think this kind of class in general desperately needs better documentation/notebooks because without it, it is hard for me to look more carefully into what the code is doing/supposed to do.

It would also help users in understanding how to handle event files more easily, I worry that most users (my past self included) will just look at the notebooks and think "I can throw in an event file when I need my timing data and it'll be fine".

[matteobachetti]

@matteolucchini1 thanks for the review! I will work on the API docs, I do realize now that I could have done much better 😅 . I'm just not sure about notebooks or user-facing documentation: this is a function that is more for internal use in the library, something to speed up the loading of small parts of big files.
This will be used by user facing classes (something like LazyLoadedEventLists, gotta find a better name but that's what they will be), but probably not directly from users.

[matteolucchini1]

@matteobachetti Ah fair enough. I do still think it would be nice to show different ways of loading event files (e.g. here's what happens when you load an heasoft-compatible file, or a generic fits file like in this PR, or whatever other source is supported), but maybe a bunch of that is beyond the scope of the PR. I'll try to dig into the code between today and early next week!

[matteobachetti]

@matteolucchini1 I wrote a small motivation tutorial here: StingraySoftware/notebooks#107

[matteolucchini1]

I really like that tutorial! I left a couple more minor comments otherwise it's getting close :)

[matteolucchini1]

No more comments! Just conflicts that need resolving now