Use np.sum in PauliString expectation display
Use np.sum in PauliString expectation display

This gave a 40% speedup.
balopat/Cirq,quantumlib/Cirq,quantumlib/Cirq,balopat/Cirq,balopat/Cirq,quantumlib/Cirq,quantumlib/Cirq,quantumlib/Cirq