Add Cifford.from_label method

[chriseclectic]

Summary

Adds Clifford.from_label method.

This functions like Operator.from_label but only for single-qubit clifford gates I,X,Y,Z,H,S.

Todo

• Add tests
• Add release note

[jaygambetta]

i agree with adding this but i think you need to change the reverse from to_dict to to_label

[chriseclectic]

@jaygambetta there is no reverse to this function, it is a convenience for initializing simple tensor-product Clifford states.

There are already to_dict and from_dict methods that can serialize/deserialize Clifford as a python dict object. These are very different to the single-qubit gate string labels. Eg Clifford.from_label('I').to_dict() will return {"stabilizer": ["+Z"], "destabilizer": ["+X"]}