fix: use left join when adding tables instead of inner join

[ManonMarchand]

This fixes #3197

The behavior that people expect is not the one that we get with the default inner join:

from astroquery.simbad import Simbad
simbad = Simbad()
simbad.add_votable_fields("mesdistance")
simbad.query_object("vega")

with inner join

No measurement --> no result

WARNING: NoResultsWarning: The request executed correctly, but there was no data corresponding to these criteria in SIMBAD [astroquery.simbad.core]
<Table length=0>
main_id    ra     dec   coo_err_maj coo_err_min coo_err_angle coo_wavelength coo_bibcode mesdistance.bibcode ... mesdistance.mespos mesdistance.method mesdistance.minus_err mesdistance.minus_err_prec mesdistance.plus_err mesdistance.plus_err_prec mesdistance.qual mesdistance.unit matched_id
          deg     deg       mas         mas          deg                                                     ...                                                                                                                                                                                   
 object float64 float64   float32     float32       int16          str1         object          object       ...       int16              object              float64                  int16                  float64                  int16                 str1            object        object  
------- ------- ------- ----------- ----------- ------------- -------------- ----------- ------------------- ... ------------------ ------------------ --------------------- -------------------------- -------------------- ------------------------- ---------------- ---------------- ----------

with left join

No measurement --> masked data

<Table length=1>
 main_id         ra              dec       coo_err_maj coo_err_min coo_err_angle coo_wavelength     coo_bibcode     mesdistance.bibcode ... mesdistance.method mesdistance.minus_err mesdistance.minus_err_prec mesdistance.plus_err mesdistance.plus_err_prec mesdistance.qual mesdistance.unit matched_id
                deg              deg           mas         mas          deg                                                             ...                                                                                                                                                                
  object      float64          float64       float32     float32       int16          str1             object              object       ...       object              float64                  int16                  float64                  int16                 str1            object        object  
--------- ---------------- --------------- ----------- ----------- ------------- -------------- ------------------- ------------------- ... ------------------ --------------------- -------------------------- -------------------- ------------------------- ---------------- ---------------- ----------
* alf Lyr 279.234734787025 38.783688956244     3.51118     2.81205            90              O 2007A&A...474..653V                     ...                                       --                         --                   --                        --                                    NAME Vega

Other changes

This PR also adds a new section in the documentation that tries to explain measurement tables in SIMBAD.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 68.34%. Comparing base (6b0af57) to head (3e9ff61).
Report is 324 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #3199   +/-   ##
=======================================
  Coverage   68.34%   68.34%           
=======================================
  Files         231      231           
  Lines       19190    19190           
=======================================
  Hits        13116    13116           
  Misses       6074     6074           

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[keflavich]

Looks good technically. I have a question about how one figures out the parameter names.

[bsipocz]

Thanks!