Fix handling of multi-model PDB without MODEL 1

[speleo3]

Fixes #95

[codecov]

Codecov Report

Merging #96 (3632f6e) into master (810750c) will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master      #96   +/-   ##
=======================================
  Coverage   72.40%   72.41%           
=======================================
  Files          24       24           
  Lines        4142     4143    +1     
=======================================
+ Hits         2999     3000    +1     
  Misses       1143     1143           

Impacted Files	Coverage Δ
propka/molecular_container.py	91.30% <100.00%> (+0.07%)	⬆️

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