Be more memory efficient converting SummedOp to MatrixOp (Qiskit/qiskit-aqua#1129)
Be more memory efficient converting SummedOp to MatrixOp (Qiskit/qiskit-aqua#1129)

Before, all summands were converted to MatrixOp and then summed. This
PR instead converts them one by one and accumulates the result,
thereby avoiding building a large intermediate list.

Closes Qiskit/qiskit-aqua#1027

* Add release note

Co-authored-by: Manoel Marques <726dfbbf2d82742fc601e72b7538ea72c4fcf7b0@us.ibm.com>
QISKit/qiskit-sdk-py,QISKit/qiskit-sdk-py,QISKit/qiskit-sdk-py