0.2.1

New Features

• Measurement samples can be returned as bitstrings instead of integers in nested lists.

• 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).

Upgrade Notes

• Adds support for multi-qubit sampling in measurements. Measurement.sample() and
  Measurement.expval() are updated to accept a sequence of qubit indices to sample, sampling
  all measured qubits if none are given.

  circuit = Circuit(2, 2)
  
  with circuit.context as (q, c):
      ops.X(q[0])
      m = ops.Measurement(q) | c
  
  simulate(circuit)
  
  m.sample(num_samples=3, as_bitstring=True)
  # ['10', '10', '10']

• 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.