Change from Pool to ProcessPoolExecutor

[PaulJonasJost]

Changed to ProcessPoolExecutor according to "concurrent.futures" documentation.

I have the indexing in since we are not using imap anymore. Do we need a tests that sees we are returning them in the right oder? Probably not necessary to me, as we are anyways sorting them shortly after.

Closes #1499?

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Codecov Report

:white_check_mark: All modified and coverable lines are covered by tests. :white_check_mark: Project coverage is 84.28%. Comparing base (bc7d89a) to head (b93064e). :exclamation: Your organization needs to install the Codecov GitHub app to enable full functionality.

Additional details and impacted files

@@             Coverage Diff             @@
##           develop    #1664      +/-   ##
===========================================
- Coverage    84.37%   84.28%   -0.09%     
===========================================
  Files          164      164              
  Lines        14320    14324       +4     
===========================================
- Hits         12082    12073       -9     
- Misses        2238     2251      +13     

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