changing the default :initarg of channel-qvm to be valid

[sophiaponte]

#210

The channel-qvm expects its :noise-model slot to be a noise-model object, but it's default initarg is nil. This causes errors when a channel-qvm is created without a defined noise-model argument. To fix this I:

• changed the default noise-model initarg of channel-qvm to be a noise-model with an empty set of noise-rules

• I added a check to initialize-instance to ensure that the noise-model slot of channel-qvmis of typenoise-model`

• added checks to a test in tests/channel-qvm

[rigettizach]

changed the default noise-model initarg of channel-qvm to be a noise-model with an empty set of noise-rules

Is there any risk this could trip up someone who meant to provide a non-empty noise model but forgot?

[sophiaponte]

Is there any risk this could trip up someone who meant to provide a non-empty noise model but forgot?

Yes, I suppose if someone forgot to provide a noise model, the channel-qvm would run with the default empty noise model as if there were no noise.

So I guess an alternative option is to make the noise model a required argument to the qvm -- that way, the noise model will only be empty if the user explicitly wants it so.

[appleby]

Normally I would lean towards Zach's suggestion to make noise-model a required argument, but all the other noisy qvm types seem to default to "empty noise" if a caller doesn't provide any noise, so I think it's reasonable for channel-qvm to follow suit here.