[MISC] consistently use "directory" instead of "folder" as a term

[sappelhoff]

closes #1039

Ideally we would add a CI step that checks for the [fF]older[s]{0,1} regex.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 71.50%. Comparing base (7ec7450) to head (e3eef7c).
Report is 1435 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1044      +/-   ##
==========================================
+ Coverage   70.53%   71.50%   +0.96%     
==========================================
  Files           9        9              
  Lines         930      930              
==========================================
+ Hits          656      665       +9     
+ Misses        274      265       -9     

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[yarikoptic]

+1 on consistency!
This PR reminded me about #962 where I unified "file name" into "filename". Now, looking at all "directory name"s and "subdirectory name", I wonder if that was the right decision ;-) : "directoryname" is not good. "foldername" also ugly, "subfoldername" is really needing German expertise.

[effigies]

Looks good modulo preceding comment.