Implement iterating/collecting PauliSumOp coefficients #5547

[paolob67]

Summary

Fixes #5547

• Added the coeffs property to return the operator coefficients as a numpy array
• Added a matrix iterator by leveraging the same code used in the PauliSparseOperator
• Added tests for the iterator

Details and comments

This fix provides the ability of being able to iterate over, or return a list of coefficients in PauliSumOp. At present, one has to dig into the details to construct the coefficients for each term. In particular, it requires multiplying two numbers for each coeffcient. There are use cases, for instance when bounding the eigenvalues of the operators.
The coeffs property returns a numpy array of the operator obtained by multiplying the coeffs of the primitive by the coefficient provided in the constructor.
The matrix iterator iterates through the Pauli table representing the the PauliSumOp in either sparse or not fashion.

[CLAassistant]

All committers have signed the CLA.

[ikkoham]

Very nice. Just a small comment.

[paolob67]

The Qiskit.qiskit-terra (Python_Tests Tutorials Python38) pipeline is failing after the commit on the PR but I really don't know how to fix that. Any help is appreciated. Thanks.

[jlapeyre]

pipeline is failing after the commit on the PR

It may be unrelated to this PR. There is probably a way to re-run it.

[kdk]

pipeline is failing after the commit on the PR

It may be unrelated to this PR. There is probably a way to re-run it.

I triggered a re-run and it passed. Maybe a transient azure issue?