[UnitaryHack] Mixed state unitary evolution and expectations

[hongyehu]

Implement mixed state quantum evolution, measurement, and expectation values. In the current implementation, the stabilizer state class has an attribute stabilizer_state.r which labels how many stabilizer generators are used to describe the state. For example, for a N qubit stabilizer state, in the gs matrix, the first N rows denote the stabilizer generators in binary vectors, and the second N rows denote the destabilizer generators in binary vectors. And the stabilizer generators used are from stabilizer_state.r to N, while destabilizer generators used are from stabilizer_state.r+N to 2N.

Therefore, if stabilizer_state.r=0, the state is fully stabilized by the first N generators, and the state is pure. And one need to change the evolution, measurement, calculation of expectation functions accordingly to general 0<stabilizer_state.r<=N for the mixed states.