Make StateMachine an interface/ABC class

[Elan456]

• We need an abstract base class version of the state machine so that state machines can be used more interchangeably in some of the other modules, including simulation.

My proposal
Do #30 and then put a IStateMachine class definition within StateMachine.cpp

[mcschoo]

I'm aiming to have #30 and #25 done by eod tomorrow - were you looking to cover this one here, or should I knock it out in the same run?