New set of rates in two-qubit errors when partner was lost #285

#284 adds marginalization over rates in depolarize2 and pauli_error2. This is not fully accurate still, however: when a qubit undergoes a two-qubit gate and its partner was lost prior to the gate, it experiences different dynamics. Therefore, the rates at which the leftover qubit experiences a Pauli error can differ from the marginal probabilities.

We should:

• Allow setting these 3 additional rates at least in the two-qubit Pauli error.
• Make the default the marginal probabilities.