Add the ability to set error probabilities per channel

[stephendiadamo]

Currently when one sets the error probability for an X or Z error in the Network, it is applied to each channel the same way. This should be a property of only the channel rather than the network and should be moved there. A consequence of this should be that, if this feature is desired, the user should create their channels during the network setup phase and use it as a parameter to the add_connection method of a host. If no such parameter is set, the channel should be noiseless.