Update SetupUnit phase detection

[IAlibay]

Currently this requires hardcoded keys in the ChemicalSystem. We should update the code to work off checking instances of each Component class.

[ijpulidos]

The pieces of code that should be relevant here are basically the hidden methods _check_states_compatibility and _detect_phase in the SetupUnit.

[ijpulidos]

While we are still not automatically detecting the phase, the changes in #106 should solve this. We are now relying on underlying gufe components functionality and typing, rather than hardcoded keys in the chemical system.

The user decides what's the "leg"/phase in the thermodynamic cycle that makes sense for them and creates the chemical system accordingly. I think this should be enough for basic usability in a first iteration.