SummedOp: equals should capture sums of matrices

[Cryoris]

What is the expected enhancement?

>>> from qiskit.aqua.operators import MatrixOp
>>> op = MatrixOp([[1, 0], [0, 0]])
>>> (2 * op) == (op + op)
False  # should be True

This is done for PauliOps as part of #1059 but should be extended to MatrixOps.
One possible implementation is to adapt SummedOp.collapse_summands to sum up all MatrixOps to one.

[romainfd]

Hey @Cryoris, could I give this a try?