Labelling peptides

https://www.degruyterbrill.com/document/doi/10.1351/pac199870010117/html

Diastereotopicity is quite complex and we should apply those rules

Strategy would be:
- add implicit hydrogens
- add missing chirality
- label NCC(=O) and determine where is N-terminal and C-terminal. Need to keep in mind the cyclic peptides
- calculate proR and proS ?
- create a method that walk the side chain ?

No files, tests, or entry points are named. Start by reading the linked diastereotopicity reference and inspecting the existing peptide and stereochemistry utilities; clarify how implicit hydrogens, missing chirality, terminal residues, cyclic peptides, and proR/proS labels should be represented. Done requires an agreed scope and tests covering those cases.

Written by the indexing model from the issue text.