Add get_zeropoint and more flexible metadata querying to SVO FPS

[keflavich]

There is metadata on SVO FPS that is inaccessible but should be.

#3528 showed the right mechanism to retrieve metadata, but it didn't allow querying on PhotCalID - while data_from_svo allowed specification of this parameter, the mechanism we are using to parse the votable drops the relevant data.

This PR adds a new get_zeropoint mechanism to very explicitly and cleanly retrieve zeropoints and also generalizes get_filter_metadata.

Note that I have made some serious mistakes in recent publications because I failed to realize the default zeropoint was vega, so it's high importance for me to expose the get_zeropoint method with an explicit mag_sys keyword to users.

[codecov]

Codecov Report

❌ Patch coverage is 90.00000% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 72.72%. Comparing base (b1cae75) to head (bb82061).
⚠️ Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
astroquery/svo_fps/core.py	90.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #3545      +/-   ##
==========================================
+ Coverage   72.66%   72.72%   +0.06%     
==========================================
  Files         219      219              
  Lines       20480    20489       +9     
==========================================
+ Hits        14882    14901      +19     
+ Misses       5598     5588      -10     

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[cgobat]

Nice. I like it.