Load constant pointing in TableLoader

[maxnoe]

This adds the possibility to read the constant pointing stored per observation block introduced in #2438 in the TableLoader.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

❗ No coverage uploaded for pull request base (main@e4402ad). Click here to learn what that means.
Report is 4 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #2481   +/-   ##
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  Coverage        ?   92.51%           
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  Files           ?      234           
  Lines           ?    19996           
  Branches        ?        0           
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  Hits            ?    18500           
  Misses          ?     1496           
  Partials        ?        0           

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[Tobychev]

Changelog should mention the new option of join_allow_empty, and codecov is probably right that there should be some test coverage of this new feature