Create tests for mcfgen.py

## Is your feature request related to a problem? Please describe.
The mcfgen script is not currently being automatically tested in any form. 

## Describe the solution you'd like
It'd be good to include unit tests for several PDB files with species of different topologies (i.e. with rings, united atom, all atom, zeolites). The resulting MCF in these tests should be compared to accepted MCF.