Support getting counts, unitary, state vector and snapshot per circuit index.

[jaygambetta]

What is the expected enhancement?

Currently it is not possible to grab the results from a Result object by the index of the circuit/experiment. That is to say the following does not work:

from qiskit import *
circs = []
for k in range(5):
    q = QuantumRegister(2, "q")
    c = ClassicalRegister(2, "c")
    qc = QuantumCircuit(q,c)
    qc.h(q[0])
    qc.cx(q[0], q[1])
    qc.measure(q,c)
    circs.append(qc)

sim = Aer.get_backend('qasm_simulator')
res = execute(circs, sim).result()
res.get_counts(3)

QISKitError: 'No counts for circuit "3"'

It should be possible to do this.