Schematize MPro-specific features; remove hardcodes from library

[dotsdl]

The fah-xchem library, and the additions coming in #155 to integrate fah_prep, feature many hardcodes specific to Mpro.

We should attempt to generalize away from these hardcodes. One approach would be to separate the operations each hardcode perform from the objects on which they operate, and place those objects in a schematized form. The schema for Mpro could be defined within the library in its own module for a start, or as part of a test suite. Other targets could be added over time, but don't need to be defined in the library to make use of the operations.