Added a method to HeasarcClass that returns the number of rows in a named catalog

[DavidT3]

A small convenience method has been included in the HeasarcClass class which takes the name of a catalog as an argument and passes it to a simple ADQL query that uses COUNT(*) to fetch the number of rows it contains.

The integer number of rows is then extracted from that return, and passed back to the user.

This abstracts another use of ADQL away from the user.

A simple test was added, and an example in the Heasarc sub-module docs.

[codecov]

Codecov Report

❌ Patch coverage is 25.00000% with 3 lines in your changes missing coverage. Please review.
✅ Project coverage is 72.65%. Comparing base (b1cae75) to head (6ae40c1).
⚠️ Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
astroquery/heasarc/core.py	25.00%	3 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #3549      +/-   ##
==========================================
- Coverage   72.66%   72.65%   -0.01%     
==========================================
  Files         219      219              
  Lines       20480    20484       +4     
==========================================
+ Hits        14882    14883       +1     
- Misses       5598     5601       +3     

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