api.add_variable() add parameter keyword

[AlexDo1]

Adds the optional parameter keyword to api.add_variable() to make it possible to link a variable to a keyword when a new varible is added.

Closes #303

[codecov]

Codecov Report

Merging #304 (d993a47) into main (ca7b775) will increase coverage by 0.33%.
The diff coverage is 27.27%.

❗ Current head d993a47 differs from pull request most recent head 824d9ae. Consider uploading reports for the commit 824d9ae to get more accurate results

@@            Coverage Diff             @@
##             main     #304      +/-   ##
==========================================
+ Coverage   64.04%   64.38%   +0.33%     
==========================================
  Files          75       75              
  Lines        3969     3931      -38     
==========================================
- Hits         2542     2531      -11     
+ Misses       1427     1400      -27     

Flag	Coverage Δ
e2e	64.38% <27.27%> (+0.33%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

Files	Coverage Δ
metacatalog/api/find.py	63.78% <33.33%> (-0.35%)	⬇️
metacatalog/api/add.py	50.45% <25.00%> (-1.21%)	⬇️

... and 2 files with indirect coverage changes

[mmaelicke]

I will approve, but I have a few minor questions