EUCLID: update de method get_spectrum to retrieve multiple spectra

[cosmoJFH]

Dear Astroquery team,

Currently the astroquery.euclid.get_spectrum() method allows one to retrieve a single spectrum associated with a given object id (as a string).

This method downloads the spectrum and stores it in a FITS file inside a (created) directory that is written locally. The output of this method is a list with the path to this FITS file, e.g.: ['<local_dir>/temp_20250327_093710/SPECTRA_RGS-sedm 2707008224650763513.fits']

We would like to be able to retrieve multiple spectra data in one single request via Astroquery. In this PR, the input parameter ids accepts a <source_id> or a designation (<data-release>+blank+<source_id>) or multiple values separated by commas or a list. In the case multiple values are used and the output_file is not set, the returned files are saved in a zip file named get_spectra_output.zip. The method always returns a dictionary where the key is the name of the returned fits file and the value is a list of Astropy Tables made from the fits file (similar to the Gaia datalink method)

cc @esdc-esac-esa-int
jira: EUCLIDMNGT-1457

[codecov]

Codecov Report

❌ Patch coverage is 76.78571% with 13 lines in your changes missing coverage. Please review.
✅ Project coverage is 73.11%. Comparing base (6a3c231) to head (e4aeb53).
⚠️ Report is 35 commits behind head on main.

Files with missing lines	Patch %	Lines
astroquery/esa/euclid/core.py	76.36%	13 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #3570      +/-   ##
==========================================
+ Coverage   73.09%   73.11%   +0.02%     
==========================================
  Files         219      224       +5     
  Lines       20592    20873     +281     
==========================================
+ Hits        15052    15262     +210     
- Misses       5540     5611      +71     

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[cosmoJFH]

We have found an issue in the documentation, due to the changes introduced in PR #3558: 7 doctests are not displayed since we included a comment after the "doctest-skip::" For example,

.. doctest-skip::
.. Skip testing as the example requires a lot of time to download a huge file

is not correct. The comment must be included before. We have included the changes in this PR. Would you rather include these changes in a separate PR?