Add alternative state preparation error implementation via the garbage state

[HGSilveri]

Now that the garbage (aka "x") state is supported in QutipEmulator, we can emulate state preparation errors by preparing the initial state in a classical mixture of $\rho = (1-\eta) \ket{g}\bra{g} + \eta ) \ket{x}\bra{x}$.

• This will allow us to do a single emulation run by solving the master equation and get a density matrix at the end.
• It is unclear how the performance will compare with the existing implementation

For now, it seems sensible to select which implementation to use based on the NoiseModel:

1. If runs and samples_per_run are defined and with_leakage=False, use the current implementation.
2. Otherwise, use the new implementation.