Sage monatomics issues in PDB loading

[Yoshanuikabundi]

While testing ion loading with Pablo, I've found two issues with the legacy PDB loader when dealing with monatomics:

1. The CCD residue name for Iodide is "IOD", but we use "I". I think "I" is likely commonly used out in the wider comp chem world too. In CCD land, "I" refers to the unusual ribonucleotide inosinic acid. I'm not sure that this needs fixing, but it might be nice to make "IOD" an alternative to "I" to catch spec-compliant PDB files. Currently they do load correctly, but if the PDB file breaks somewhere else, it appears in the diagnostic.
2. Xenon is missing from our substructure library.