Add ADP+Pi state

[blairlyons]

Use Case

It would be nice if, in addition to ATP and ADP states, actin monomers also had a ADP+Pi state.

Acceptance Criteria

Actin monomers can be in either ATP, ADP+Pi, or ADP states.

Details

The new ADP+Pi state should work similarly to the current ATP/ADP states, affecting reaction rates but not geometric constraints.

This state is important for biological function, so should be included in the models.