Remove unused imports
Code quality fixes
Use the new APIs for molecular weight calcs.
Update documentation
Minor quality fixes
Prefer using the new List<> based API points in the interface defaults.
Isotopes there is a long tail, we can use stats from PubChem to have a small table of common isotopes.
For a true singleton, we should use an enum. To handle the exception we stash it and allow referring back to it later.