Rename QubitRegister numQubits field

[TysonRayJones]

Since this is the number of qubits suggested by the dimensions of the state-vector, which is twice too many when the register is storing a density matrix (and should instead refer to numDensityQubits).

Consider renaming

• numDensityQubits -> numQubitsRepresented
• numQubits -> numQubitsInStateVec

Still seems hacky and gross ¯\(ツ)/¯

[TysonRayJones]

The above renaming has been done in the density-matrix branch, though could be improved.
Operations performed on statevecs (prefixed with statevec_) consult qureg.numQubitsInStateVec.
Operations on general QubitRegisters consult qureg.numQubitsRepresented