Fermi pulsar catalogs base class

[MRegeard]

Split of PR #4699. Here is the base class part.

This introduces a base class for object of the pulsar catalog of Fermi (both 2PC and 3PC).

The part where I modify the core class SourceCatalog could be extracted into another base class, say SourceCatalogPC, but it will add another level of inheritance. I don't have a strong opinion on that.

[codecov]

Codecov Report

Attention: Patch coverage is 30.61224% with 68 lines in your changes are missing coverage. Please review.

Project coverage is 75.11%. Comparing base (6616660) to head (663e79e).
Report is 313 commits behind head on main.

Files	Patch %	Lines
gammapy/catalog/fermi.py	20.00%	56 Missing ⚠️
gammapy/catalog/core.py	57.14%	12 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #5056      +/-   ##
==========================================
- Coverage   75.24%   75.11%   -0.13%     
==========================================
  Files         232      234       +2     
  Lines       34318    34944     +626     
==========================================
+ Hits        25822    26248     +426     
- Misses       8496     8696     +200     

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[QRemy]

Thanks @MRegeard, maybe some of the code in the base class could be moved in private functions that can be redefined in the sub classes.

[bkhelifi]

Thanks @MRegeard ... In addition to my small comments, there is no dedicated test to these new functionalities?

[Astro-Kirsty]

Thanks for the effort @MRegeard.
Do some tests need to be added for this?

Also a correction throughout: dictionnary --> dictionary

[registerrier]

Thanks @MRegeard . This is nice. I have left a number of comments inline

[registerrier]

This does not seem necessary. The result is longer than the direct call.
If this is there for daughter classes, maybe it could be a simple attribute rather than a function?

[MRegeard]

I honestly cannot remember why I did that.

[registerrier]

The code is very similar between _lookup_spectral_source_idx and _lookup_extended_source_idx. It could be turned a single function taking either self.spectral_table[source_name_key] or self.extended_sources_table[self._source_name_key]as argument

[MRegeard]

Thanks for the effort @MRegeard. Do some tests need to be added for this?

Also a correction throughout: dictionnary --> dictionary

@Astro-Kirsty, I don't think test are needed because the base class is never other than by the daughter classes. So the tests happen in the daughter class. That is also the scheme for the other Fermi base class.

[registerrier]

Thanks @MRegeard . Looks good to me.