Expectation Value snapshot extension

[chriseclectic]

What is the expected behavior?

This will be used by various methods of the QasmSimulator to return snapshots of expectation value outcomes <M> for some operator M.

Eventually we will allow different input classes for M but for now we will allow N-qubit Operator objects (which can be constructed from other Operators, lists, np.arrays, Paulis, etc). Behind the scenes these should be converted into either pauli expectation value snapshots, or dense matrix snapshots.

The qobj format for Pauli expectation value snapshot params is a list [[coeff, pauli], ...] where coeff is a complex number (JSON: [re, im]), and pauli is a string containing only IXYZ for the Pauli term (qubit 0 to N-1 are characters right to left).

class SnapshotExpectationValue(Snapshot):
    __init__(self, label, op, pauli=True, variance=False):
        # num_qubits can be inferred form size of operator.
        # If Pauli is true convert to Pauli expval snapshot
       # type: "expectation_value_pauli" or ""expectation_value_pauli_with_variance"
       # otherwise we use general matrix ones
      # type: "expectation_value_matrix" or ""expectation_value_matrix_with_variance"
      # See `operations.hpp` for input format

# Monkey patch function
def snapshot_expectation_value(self, label, op, qubits=None, variance=False):
     # If qubits is None snapshot all qubits.

circuit.snapshot_expectation_value = snapshot_expectation_value