Bug fix: SEQRES serNum digits were extended in PDB v3.2 (2008)

* Bug fix: SEQRES serNum digits were extended in PDB v3.2 (2008).
  Thanks to a researcher who reports the patch via email.
Refactored to shorten, remove rescues, and clarify.
Fixed the stack level too deep errors by using Hash#invert.
Refactor Bio::AminoAcid#weight: Early return clearer and idiomatic.