Pauli gadget support

[y-richie-y]

Currently the library only supports ZX phase gadgets and CNOT layers. It would be great to support arbitrary Pauli gadgets and Clifford layers.

• Pauli gadgets class
• Clifford layer class
• Drawing interface
• Pauli gadget extraction
• ZX representation?

Reference on Pauli gadgets: https://arxiv.org/pdf/1906.01734.pdf