Add support for counter-poise calculations

[awvwgk]

• verify results with mctc-gcp program
• add C API and API documentation
• compute virial in all routines
• check periodic boundary condition support
• cleanup gcp code

Closes #120

[codecov]

Codecov Report

Attention: Patch coverage is 69.93218% with 399 lines in your changes missing coverage. Please review.

Project coverage is 54.42%. Comparing base (3a50cbd) to head (f746450).
Report is 35 commits behind head on main.

Files with missing lines	Patch %	Lines
src/dftd3/gcp.f90	85.38%	67 Missing and 35 partials ⚠️
app/driver.f90	1.17%	80 Missing and 4 partials ⚠️
src/dftd3/gcp/param.f90	77.77%	30 Missing and 54 partials ⚠️
app/cli.f90	0.00%	73 Missing and 1 partial ⚠️
src/dftd3/output.f90	0.00%	36 Missing ⚠️
src/dftd3/api.f90	66.07%	3 Missing and 16 partials ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #116      +/-   ##
==========================================
+ Coverage   48.63%   54.42%   +5.78%     
==========================================
  Files          34       27       -7     
  Lines        3707     4171     +464     
  Branches     1518     1644     +126     
==========================================
+ Hits         1803     2270     +467     
- Misses       1207     1215       +8     
+ Partials      697      686      -11     

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