Add fix for reading poscar reading

[awvwgk]

Closes #60
Closes dftd4/dftd4#207

[codecov]

Codecov Report

Attention: Patch coverage is 58.33333% with 10 lines in your changes missing coverage. Please review.

Project coverage is 69.78%. Comparing base (77f65c6) to head (6ff2c52).
Report is 1 commits behind head on main.

Files	Patch %	Lines
test/test_read_vasp.f90	66.66%	0 Missing and 7 partials ⚠️
src/mctc/io/read/vasp.f90	0.00%	0 Missing and 3 partials ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #64      +/-   ##
==========================================
- Coverage   69.79%   69.78%   -0.01%     
==========================================
  Files          64       64              
  Lines        8618     8639      +21     
  Branches     2579     2586       +7     
==========================================
+ Hits         6015     6029      +14     
- Misses        782      783       +1     
- Partials     1821     1827       +6     

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