Additional conversions

[hechth]

• [x] CTS: casno -> InChI
• [x] CTS: HMDBID-> InChI
• [x] PubChem: InChI -> PubChemID
• [ ] PubChem: InChI -> HMDBID
• [x] PubChem: PubChemID -> HMDBID
• [x] PubChem: PubChemID -> InChI
• [x] IDSM: iupac_name -> InChI
• [x] BridgeDb: HMDB <-> PubChemCID <-> ChemSpider <-> Wikidata <-> ChEBI

[hechth]

Since the BridgeDb has already a quite homogenous WebAPI, we can basically use a single function and the identifier mapping for the implementation I think (https://bridgedb.github.io/pages/system-codes.html) and WebService