ENH: Add seg entity to segmentations and time activity curves

[mnoergaard]

This PR updates the naming of the output segmentations and corresponding time activity curves to have the 'seg' entity instead of this information being allocated to the 'desc' entity. This will make the naming of files more clear, and it will be absolutely clear from the time activity curves, that they were derived using a specific segmentation 'seg' and with the preproc being added to the 'desc' entity, reflecting that these are preprocessed data.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 78.08%. Comparing base (4752269) to head (e49b27e).
Report is 5 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main      #54   +/-   ##
=======================================
  Coverage   78.08%   78.08%           
=======================================
  Files          81       81           
  Lines        6027     6027           
  Branches      625      624    -1     
=======================================
  Hits         4706     4706           
+ Misses       1169     1166    -3     
- Partials      152      155    +3     

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