Refactor `MBQCircuit` and `GraphState`

[kennethbruskiewicz]

Although flow is a property of graph states but graph states can be primed without input or output edges prepared, there are functions in MBQCircuit like inputc and outputc that could make the flow algorithms easier to write and understand. Between MBQCircuit, GraphState and Flow there appear to be a lot of shared responsibilities. Refocusing on how MBQCircuit should be used in practice should help us find ways to refactor these three classes by pushing more detail away from the user.