Optimise code for when noise_model = None

[garymooney]

Currently, the quantum simulator always simulates and samples the density matrices of the circuits twice. The first with no noise model for the exact values and then again with the noise model for the noise information. If the noise model is None, the intention is to process the circuit and render the qMuVi without including any noise information.

At the moment, if the noise model is None, both simulations are still computed, where the second has an empty noise model. The second simulation is redundant, so it can be removed in this case. Also, a full density matrix simulation does not need to be computed when there is no noise, a statevector is sufficient. Sample the statevectors instead of the density matrices and use statevectors throughout to save memory, enabling larger circuits to be computed.

Implement any other related optimisations that might be found on the way.