State of Instances

[codygunton]

We should simplify the mental model of instances. This includes the use of is_accumulator (probably there should just be an accumulator member somewhere to replace this). It also includes the fact that in the ProverInstances abstraction objects are only initialized to varying degrees, and quantities that are supposed to fixed across all instances in the class are only assumed to be (this isn't checked or otherwise ensured by the code).