Add COSMO implicit solvation model

[awvwgk]

• use ddCOSMO library to solve COSMO equations

[ghutchis]

Looks great! Is it possible to get the solvent volume / surface area printed as computed properties (or via the Python API). I'd love to use XTB / CREST to augment solubility predictions.

[awvwgk]

See #336 for the extension of the C-API w.r.t. to solvation. I have not yet decided on the necessary API objects and calls to implement this, but it is on the list.

[ghutchis]

I also saw in the documentation about:

$write
   gbsa=true

This looks like it would give what we want - grabbing the SASA (even h-bond terms).

[awvwgk]

@ghutchis I always advise against parsing standard output from xtb, save yourself the pain to decrypt the ascii table and just add the terms you are interested in to the JSON dump.
Also, we should probably move this discussion to a new issue than...

[codecov]

Codecov Report

Merging #274 (c011eb8) into master (f6b888f) will increase coverage by 0.97%.
The diff coverage is 86.29%.

@@            Coverage Diff             @@
##           master     #274      +/-   ##
==========================================
+ Coverage   40.79%   41.77%   +0.97%     
==========================================
  Files         304      307       +3     
  Lines       50792    51721     +929     
==========================================
+ Hits        20722    21605     +883     
- Misses      30070    30116      +46     

Impacted Files	Coverage Δ
src/prog/main.F90	40.35% <0.00%> (-0.73%)	⬇️
src/solv/input.F90	0.00% <ø> (ø)
src/set_module.f90	27.85% <33.33%> (+0.01%)	⬆️
src/solv/model.f90	55.59% <81.81%> (+4.38%)	⬆️
src/solv/ddcosmo/core.f90	84.04% <84.04%> (ø)
src/solv/ddcosmo/solver.f90	84.26% <84.26%> (ø)
src/solv/cosmo.f90	94.31% <94.31%> (ø)
TESTSUITE/gfn1.f90	100.00% <100.00%> (ø)
TESTSUITE/gfn2.f90	77.74% <100.00%> (+2.19%)	⬆️
TESTSUITE/tests_peeq.f90	94.26% <100.00%> (+0.09%)	⬆️
... and 10 more

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