Code refactoring

[dlasecki]

Summary

Introduces a little bit of structure to second quantized operators generated by problems.

Details and comments

It deals with a bad practice of addressing the main operator via an index. In the future we might want to think about introducing even more structure by subclassing for example, depending on the problem at hand.
Let me know if you have better ideas for naming.

[woodsp-ibm]

Since the code is now released changes to public interfaces need to be deprecated. So for example it seems to me for instance a new method would need to be added to replace the old one that returns a List whereby that one is deprecated and the user is referred to the new.

Since there is also discussion in changing VQE (ie min eigen solvers) - to say allow a dictionary instead of List - do we want to delay changing things until that is done since this seems to be related since ultimately these need to get passed to VQE?

[dlasecki]

Since the code is now released changes to public interfaces need to be deprecated. So for example it seems to me for instance a new method would need to be added to replace the old one that returns a List whereby that one is deprecated and the user is referred to the new.

Since there is also discussion in changing VQE (ie min eigen solvers) - to say allow a dictionary instead of List - do we want to delay changing things until that is done since this seems to be related since ultimately these need to get passed to VQE?

Sure, let's wait and see how the situation develops. Good point about deprecations - I will rely on your suggestions there.

[mrossinek]

The idea of the ProblemSecondQuantizedOps class will largely be implemented by the Property objects introduced in #220. In combination with the planned step of #243 this should mostly alleviate the need for this PR, right? @dlasecki

[dlasecki]

The idea of the ProblemSecondQuantizedOps class will largely be implemented by the Property objects introduced in #220. In combination with the planned step of #243 this should mostly alleviate the need for this PR, right? @dlasecki

Yes, I agree. Let's close this.