Avoid NoisyResults by averaging over density matrices instead

[HGSilveri]

When running simulations that require changing the Hamiltonian, we take the coherent states after each time evolution, sample them and average over the results.
In the process, we are unnecessarily discarding information that we could otherwise keep if we instead just made a final density matrix as a classical mixture of each coherent state.
This would also allow us to get rid of

• NoisyResults, having CoherentResults as the sole return type of QutipEmulator
• Get rid of NoiseModel.samples_per_run