Enhancement unroll loops

[lukasdreyer]

Closes #1915

Describe your changes here:
Adds option to unroll loops in standalone scheme.

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 76.16%. Comparing base (bb8e4e4) to head (89f799b).

Additional details and impacted files

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##           enhancement-ancestor_id    #1913   +/-   ##
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  Coverage                    76.16%   76.16%           
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  Files                          103      103           
  Lines                        19430    19430           
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  Hits                         14798    14798           
  Misses                        4632     4632           

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[Davknapp]

Thanks for the PR! We like the idea, however we prefer a more general approach that is not gcc specific (your approach currently is, if I understand it correctly).

Have a look at https://github.com/DLR-AMR/t8code/blob/main/src/t8_helper_functions/t8_unrolled_for.hxx
a pure C++ approach to unroll loops.
An example how to use it can be found in:
https://github.com/DLR-AMR/t8code/blob/main/test/t8_types/t8_gtest_vec.cxx#L338