Build Cython extensions out-of-source

[JCGoran]

Follow-up on #2662.
According to this SO answer, we can add an attribute to the Cython Extension to make it generate the c file out-of-source.
Also removes an unused setup.py.in file.

[azure-pipelines]

✔️ c6fa345 -> Azure artifacts URL

[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

[nrnhines]

Don't know what's going on behind the scenes, but it works :) . I see that the files end up in

temp$ find . -name ctng.c
./build/share/lib/python/neuron/rxd/geometry3d/ctng.c

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 67.07%. Comparing base (d68acad) to head (5a81903).
Report is 1 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #3302   +/-   ##
=======================================
  Coverage   67.07%   67.07%           
=======================================
  Files         571      571           
  Lines      111055   111055           
=======================================
  Hits        74488    74488           
  Misses      36567    36567           

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[azure-pipelines]

✔️ 5a81903 -> Azure artifacts URL