Add `witnesses_for` method on `MBQCircuit`

[kennethbruskiewicz]

When considering Pauli Flow and other problems, knowing what qubits later in time have the responsibility of correcting errors on qubits earlier in time is useful.

A collection of witness methods

• MBQCircuit.witnesses(v=None)
  • Returns some representation (set of Ments? Dict of Ments?) of all the witnesses of v. If v=None return the entire dictionary.
• MBQCircuit.witness(t, s)
  • Return True if t is a witness of s else False
• MBQCircuit.correct(v)
  • Correct the value of v and return some useful result
• MBQCircuit.witness_matrix(d=None).
  • Matrix representation of the current switch state of a circuit at evaluation depth d? If d=None, then return the witness state of the circuit after completely evaluated.

@BestQuark thoughts on these methods and their feasibility/usefulness?