[Feature Request] Use Gate Based Parser to read Pauli rotations as well

[gwwatkin]

Issue Description

#257 introduces gate based circuits. It is desirable to avoid the code duplication that there currently is between the Circuit of Pauli Rotation's parser (introduced by #254) and the Gate based parser.

Also adds a layer of abstraction between QASM and Pauli circuits, which is probably desireable.

Proposed Solution

Use the gate based parser to read QASM into a circuit of gates. Then read the circuit of gates into Pauli rotations. Probably worth breaking to Clifford+T at the gate level, as opposed to rotations.