Fix handling of hh.mod when using coreNEURON

[JCGoran]

Fixes #3000.
All mod files are first copied to the build dir (unless it's hh.mod in coreNEURON mode, in which case we modify the file) at configure time, then at build-time those files are actually converted to cpp files and compiled.

Unfortunately, one cannot use file at build-time, so we need to use add_custom_command with an external util (like sed) for replacing _reg() with _reg_() in the generated cpp files (since those are actually generated only at build-time). I also opted for using cmake -E copy etc. for modifying the file because 1) sed '[pattern]' [input] > [output] does not work, and 2) the default MacOS sed has some idiosyncrasies compared to GNU sed that I wanted to avoid.

[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

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[azure-pipelines]

✔️ 12b619f -> Azure artifacts URL

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 67.07%. Comparing base (8dfe5a3) to head (12b619f).
Report is 1 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #3301   +/-   ##
=======================================
  Coverage   67.07%   67.07%           
=======================================
  Files         571      571           
  Lines      111055   111055           
=======================================
+ Hits        74487    74488    +1     
+ Misses      36568    36567    -1     

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[nrnhines]

LGTM