enable sampling for readout error

[hhorii]

Summary

This PR adds support of sampling for read-out errors in qasm simulator.

Details and comments

Currently, qasm simulator enables sampling if

• noise_model.ideal() returns true, and
• all the measure operations are the bottom in a circuit

This PR will change as follows:

• noise_model.ideal() or noise_model.only_readout_errors() returns true, and

• all the measure and readout-error operations are the bottom in a circuit.

• Implement this feature

• Add tests

[chriseclectic]

I think it would be good to have the noise model objects to have methods that return True/False to check for readout errors, local quantum errrors, and non-local quantum errors (this will also be useful for optimisation passes later)

• noise_model.ideal(): returns True if there are no quantum errors nor readout errors, otherwise False.
• noise_model.has_readout_errors(): returns True if there are readout errors, or false if not (regardless of quantum errors present).
• noise_model.has_quantum_errors(): returns True if there are either local or non-local quantum errors (regardless of readout errors).
• noise_model.has_local_quantum_errors(): returns True if there are local quantum errors, or false if not (regardless of non-local quantum errors or readout errors present).
• noise_model.has_nonlocal_quantum_errors(): returns True if there are non-local quantum errors, or False if not (regardless of local quantum errors or readout errors present).

Then in this PR you could replace for example noise_model_.ideal() || noise_model_.only_readout_errors() with noise_model_.has_quantum_errors() == false

[chriseclectic]

This looks good @hhorii, could you fix the conflicts and update CHANGELOG and then we can merge