Allow returning eigenstates as an operator.

[Ericgig]

Problem Description

qobj.eigenstates() return the states as a list of kets, but it is something more useful to have them as a single operator where each state is a column.

Proposed Solution

Add a keyword argument (output_type?) which choose whether a list of kets or an operator is to be returned.
Eigenstates are computed as an operator, we then just have to skip the step that split them into kets.
The normalisation, phase fixing steps should still be completed.

Alternate Solutions

No response

Additional Context

Idea advanced in #2328.

[rochisha0]

Can I work on this issue? @Ericgig

[Ericgig]

@rochisha0
Yes.