Lightweight data structure for representing circuits/operators

[co9olguy]

We need a simple framework-independent Python data structure for representing quantum circuits/operators that can be used as a parser output.

This data structure should be very lightweight (likely a class containing only simple attributes (lists, dicts, etc.). Eventually, this data structure will serve as the reference implementation, to confirm that a particular implementation is consistent with the spec.

[co9olguy]

Note that simple prototypes of atomic units like gates and operators have already been created in parser.py

[josh146]

More atomic units have been added in #8