Add QChem z-matrix reader

[awvwgk]

Allows to read QChem input files specifying the geometry as z-matrix instead of cartesian coordinates.

[codecov]

Codecov Report

Attention: Patch coverage is 73.17073% with 44 lines in your changes missing coverage. Please review.

Project coverage is 69.81%. Comparing base (77f65c6) to head (ddca908).
Report is 8 commits behind head on main.

Files with missing lines	Patch %	Lines
src/mctc/io/read/qchem.f90	65.47%	6 Missing and 23 partials ⚠️
test/test_read_qchem.f90	81.25%	0 Missing and 15 partials ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #73      +/-   ##
==========================================
+ Coverage   69.79%   69.81%   +0.01%     
==========================================
  Files          64       64              
  Lines        8618     8786     +168     
  Branches     2579     2634      +55     
==========================================
+ Hits         6015     6134     +119     
- Misses        782      791       +9     
- Partials     1821     1861      +40     

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