Fix z matrix reader

[awvwgk]

Correctly account for atom indices for angles and dihedrals

[codecov]

Codecov Report

Attention: Patch coverage is 83.33333% with 3 lines in your changes missing coverage. Please review.

Project coverage is 69.85%. Comparing base (77f65c6) to head (8a54e7f).
Report is 9 commits behind head on main.

Files with missing lines	Patch %	Lines
src/mctc/io/read/qchem.f90	83.33%	0 Missing and 3 partials ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #74      +/-   ##
==========================================
+ Coverage   69.79%   69.85%   +0.05%     
==========================================
  Files          64       64              
  Lines        8618     8794     +176     
  Branches     2579     2642      +63     
==========================================
+ Hits         6015     6143     +128     
- Misses        782      786       +4     
- Partials     1821     1865      +44     

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