Checking states compatibility

[ijpulidos]

We currently have a _check_states_compatibility function that is tailored towards the protein-ligand nonequilibrium cycling flow. We should refactor it such that it can be used with different protocols, potentially enabling the use of the SetupUnit to be shared between different protocols.

This includes:

• Checking number of components in both systems
• Checking that only a single component is alchemical
• Validating that the types of components are the ones that make sense for the protocol