Add support for statistical mixtures of quantum states.

[Renmusxd]

Quantum circuits often have statistical mixtures of states (meaning there's a classical probability of having any of a set of quantum states). Implementing gates which provide classical noise and produce these mixtures can better match real life applications.

These mixtures would likely need to have a minimum probability or max number of states to prevent exponential blowup of memory usage with the number of applications of these noisy gates.