Derby Properties should report which data source provides attributes

[egonw]

Currently, users of the webservice have to guess which data source has attributes, see these two issues:

• the webservice should handle old and new HMDB identifiers bridgedb-webservice#1
• the webservice should return the same attributes, independent of the metabolite data source used BridgeDbWebservice#3

It would be nice of the Properties returned by a Derby file actually says which "root" identifiers actually have attributes. For example, for the metabolite ID mapping database, attributes are distributed over HMDB and ChEBI, see the second issue above. cc @DeniseSl22