Correctly update chain number when changing to another chain

[awvwgk]

Stale chain identifier would lead to repeated writing of TER after a chain changed.

[codecov]

Codecov Report

Merging #33 (a0029c0) into main (5db20f9) will decrease coverage by 0.01%.
The diff coverage is 0.00%.

@@            Coverage Diff             @@
##             main      #33      +/-   ##
==========================================
- Coverage   68.18%   68.17%   -0.02%     
==========================================
  Files          56       56              
  Lines        6405     6406       +1     
  Branches     1961     1961              
==========================================
  Hits         4367     4367              
- Misses        671      672       +1     
  Partials     1367     1367              

Impacted Files	Coverage Δ
src/mctc/io/write/pdb.f90	25.00% <0.00%> (-0.59%)	⬇️

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