[Feature | Performance] Add circuit module, Merge gates acting on same qubits

[dominikandreasseitz]

• Add circuit module
• Add logic to group gates and merge their operators together in case they are acting on the same qubits
• Improve docstrings and general documentation

[jpmoutinho]

Thanks @dominikandreasseitz. I think it is worth adding a test for correctness. I would suggest some series of gates including different types of single and multi-qubit gates and asserting equivalent_state with and without merging. Maybe you can make the merging optional with a boolean defaulting to True?

[jpmoutinho]

Thanks! As discussed in the call