Allow reading of v3000 connection tables

[awvwgk]

Closes #32

[codecov]

Codecov Report

Merging #34 (37cb0d0) into main (5db20f9) will increase coverage by 1.83%.
The diff coverage is 84.51%.

@@            Coverage Diff             @@
##             main      #34      +/-   ##
==========================================
+ Coverage   68.18%   70.02%   +1.83%     
==========================================
  Files          56       56              
  Lines        6405     7215     +810     
  Branches     1961     2105     +144     
==========================================
+ Hits         4367     5052     +685     
- Misses        671      685      +14     
- Partials     1367     1478     +111     

Impacted Files	Coverage Δ
src/mctc/io/read/ctfile.f90	61.80% <54.11%> (-11.43%)	⬇️
test/test_read_ctfile.f90	90.20% <92.46%> (+2.58%)	⬆️
src/mctc/io/write/pdb.f90	25.00% <0.00%> (-0.59%)	⬇️

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