Add 'entity_class_byname' field

[soininen]

The new field works pretty much like entity_byname but for entity classes.

Preparing for Pandas dataframes.

Checklist before merging

• Documentation (also in Toolbox repo) is up-to-date
• Release notes have been updated
• Unit tests have been added/updated accordingly
• Code has been formatted by black & isort
• Unit tests pass

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 81.11%. Comparing base (9d796f2) to head (7c39cbe).
Report is 2 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #481   +/-   ##
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  Coverage   81.10%   81.11%           
=======================================
  Files          76       76           
  Lines        9881     9885    +4     
  Branches     1477     1478    +1     
=======================================
+ Hits         8014     8018    +4     
  Misses       1520     1520           
  Partials      347      347           

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