Code cleanup

[keefehuang]

Major update to clean up code.

• Added circuit_gate helper function to generate CircuitGate objects.

  circuit_gate(target_wire::Int, g::AbstractGate, control_wires::Int...) 

• Removed CircuitGateChain object to remove one layer of abstraction. Now abstractions go AbstractGate -> CircuitGate -> Moments -> Circuit
• Added customized MeasurementOperator objects, these are now represented as sparse matrices to make more efficient
• Shifted number of wires out of parametrization, this prevents type stability
• Created custom _matrix function to create sparse matrices for ControlledGate objects more efficiently
• Updated apply() methods to be more type stable -> Needs future update to improve performance

[cmendl]

Can we let matrix(g) (with 'g' some gate) return a dense matrix, and rename matrix(cg::CircuitGate) to sparsematrix(cg::CircuitGate)? Why is there a separate "_matrix" function for CircuitGate?

[cmendl]

The naming "size" in size(cg::CircuitGate) is a bit confusing; maybe rename to "required_wires" or similar?

[cmendl]

"get_data" at the end of gates.jl presumably no longer required...

[cmendl]

The naming "compile.jl" is somewhat confusing, rename to "unitary2circuit.jl" or similar?

[cmendl]

circuitgate.jl, line 77: Int[i for i in cg.iwire] can be replaced by collect(cg.iwire). (There might be other occurrences as well.)

[cmendl]

One level of indentation missing at cricuit.jl, line 164

[keefehuang]

• updated sparse_matrix and related functions
• updated size to num_wires -> Also used for gates and returns Int with number of wires/qubits required
• get_data is a helper function that doesn't error when querying gates for info -> hard to differentiate between gates with no variable data etc.
• updated compile.j names
• updated collect instead of list compre.

[cmendl]

"get_data" at the end of gates.jl presumably no longer required...

can we remove get_data?