Store state in Circuit

[thisac]

Changes the way that states are stored and accessed. Instead of being returned from the simulate() function, which now returns None, the state is stored in the Circuit object. This makes more sense since the measurement results are already stored in the Circuit object, inside the classical register bits, and will hopefully cause less confusion with the state only altering the circuit and not also returning results.

• The state is now stored in the Circuit object (same as the bits/measurement results) instead of being returned by the simulate() function. It can now be accessed via the Circuit.state property.

• A density matrix representation of the state can be accessed via the Circuit.density_matrix property, for both pure and mixed states (with the former being lazily calculated from the state vector).

[codecov-commenter]

Codecov Report

Merging #30 (20ea2cf) into master (ecb8b06) will not change coverage. The diff coverage is 100.00%.

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@@            Coverage Diff            @@
##            master       #30   +/-   ##
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  Coverage   100.00%   100.00%           
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  Files           15        15           
  Lines         1191      1224   +33     
  Branches       184       191    +7     
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+ Hits          1191      1224   +33     

Impacted Files	Coverage Δ
dwave/gate/circuit.py	100.00% <100.00%> (ø)

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