Generate dynamic MPI headers out-of-source

[JCGoran]

With this change, both src and share dirs no longer contain any build artifacts 🎉 (except *.egg-info, which is a bug in setuptools, see pypa/setuptools#1347, though I hope to replace setuptools with scikit-build-core at some point in the future).

[sonarqubecloud]

Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
0.0% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarQube Cloud

[azure-pipelines]

✔️ e4ec088 -> Azure artifacts URL

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 67.07%. Comparing base (9500741) to head (e4ec088).
Report is 1 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #3304   +/-   ##
=======================================
  Coverage   67.07%   67.07%           
=======================================
  Files         571      571           
  Lines      111055   111055           
=======================================
  Hits        74488    74488           
  Misses      36567    36567           

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[nrnhines]

Excellent. I have no problem with merging as is. I notice that src/coreneuron/mpi did not have an issue with files generated in the source tree. Since that is mostly the same (derived from) as src/nrnmpi I suppose one of those will someday disappear. I have not investigated, but wonder how the coreneuron side handles the issue.