Replace scalar enumerable_thread_specific with parallel_reduce

[zfergus]

Description

Replace scalar enumerable_thread_specific with parallel_reduce.

Type of change

Please delete options that are not relevant.

• Enhancement (non-breaking change which improves existing functionality)

How Has This Been Tested?

Existing unit tests.

Checklist:

• I have followed the project style guide
• My code follows the clang-format style guidelines of this project
• I have performed a self-review of my code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• My changes generate no new warnings
• I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes
• Any dependent changes have been merged and published in downstream modules

[codecov]

Codecov Report

Attention: Patch coverage is 96.66667% with 1 line in your changes missing coverage. Please review.

Project coverage is 96.31%. Comparing base (86f1db3) to head (5a7e01d).
Report is 2 commits behind head on main.

Files with missing lines	Patch %	Lines
src/ipc/collisions/collisions.cpp	85.71%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #121      +/-   ##
==========================================
- Coverage   96.32%   96.31%   -0.02%     
==========================================
  Files         102      102              
  Lines        7602     7601       -1     
==========================================
- Hits         7323     7321       -2     
- Misses        279      280       +1     

Flag	Coverage Δ
unittests	96.31% <96.66%> (-0.02%)	⬇️

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