Fix changing `simbad` config items at runtime

[eerovaher]

Previously changing simbad config items at runtime had no effect. Now the config items are read when they are needed, unless the user has specified the corresponding class or instance variable, in which case the latter takes precedence.

Relevant for #2291

[codecov[bot]]

Codecov Report

Merging #2292 (0f6a310) into main (0883e92) will increase coverage by 0.01%. The diff coverage is 95.23%.

@@            Coverage Diff             @@
##             main    #2292      +/-   ##
==========================================
+ Coverage   62.99%   63.01%   +0.01%     
==========================================
  Files         133      133              
  Lines       17291    17300       +9     
==========================================
+ Hits        10892    10901       +9     
  Misses       6399     6399              

Impacted Files	Coverage Δ
astroquery/simbad/core.py	90.84% <95.23%> (+0.19%)	:arrow_up:

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[eerovaher]

I had to rebase to solve a conflict in the change log.

[eerovaher]

I had to rebase again to resolve another merge conflict in the change log.

I also decided to add a _row_limit property that works analogously to the other two I had added earlier. The main reason I added the properties was to avoid code duplication and adding a property for row limit was not needed for that. But for consistency and simplicity it is better if there are properties for all the config items.

[bsipocz]

I would like to get this change done in the same release for a bunch of the modules (at least the ones that are most often used), and maybe also add the warning #638 suggest (there is one already in mast.missions), so would push this to the next release. The plan is to get that out reasonably quickly, and the plan also includes quite a few backend changes, e.g. caching, cloud access, etc, so this would perfectly fit in there.

[eerovaher]

I have rebased the commits to update the change log entry.

[eerovaher]

For some reason none of the tests ran at all. Should I rebase again to start the tests?

[eerovaher]

I've updated the pull request so that the documentation about the Simbad subpackage is kept up to date regarding the configuration, and I had to rebase to resolve a merge conflict in the change log.

[eerovaher]

I had to rebase to fix a merge conflict in the change log, again.