Add support for chainIDs for PDB/MOL2 molecules

[lunamorrow]

Currently unable to add chainIDs to a Topology. If they are NULL in OpenBabel they cannot be detected as such currently as they are not equal to an empty string, 0 or None. As a result, Universe is raising an error: "ValueError: Length of chainIDs does not match number of atoms" when parsing the created Topology. Support for chainID conversion has been removed until it can be resolved properly, but this attribute needs to be supported in future within the OpenBabelParser.