Preserve PDB information when interconverting with RDKit

[chryswoods]

We'd like to add functionality to the sire.convert code so that atom and residue information is preserved when interconverting with RDKit. Currently, this information is lost, because the atom and residue names and numbers are not added to the metadata of the RDKit molecule.

The right solution would be to make use of the molFileAlias atom property to store atom names, and also use atom.SetMonomerInfo(...) to add additional metadata (e.g. AtomPDBResidueInfo) to each atom. The interconverter should populate these when converting to RDKit, and should check for these when converting from RDKit.