Fixes Qiskit/qiskit-aqua#1388 - Indexing by slices now returns the corresponding type (Qiskit/qiskit-aqua#1397)
Fixes Qiskit/qiskit-aqua#1388 - Indexing by slices now returns the corresponding type (Qiskit/qiskit-aqua#1397)

* Fixes Qiskit/qiskit-aqua#1388 - Indexing by slices now returns the corresponding type

ListOp.__getitem__() now returns `ListOp`, `SummedOp`, `ComposedOp`, or `TensoredOp` depending on the object in which it was called.

* Removed whitespace

* Changed `for` for `ddt` data on test

Co-authored-by: Steve Wood <df319c8b10182f101be29ecf0d4d148dec6c4bfc@users.noreply.github.com>
QISKit/qiskit-sdk-py,QISKit/qiskit-sdk-py,QISKit/qiskit-sdk-py