Implement SpinOp.from_polynomial_tensor

[mrossinek]

What should we add?

#665 proposes the addition of a PolynomialTensor class to handle coefficient storage.

This issue tracks the implementation of the method which can construct a SpinOp from those coefficients.
Below, I summarize the proposed structure of the PolynomialTensor.data field:

SpinOp

This class could also be used to store coefficients for the construction of SpinOps.

{
  "XYZ": [
    [
      [1, 2],
      [3, 4]
    ], [
      [5, 6],
      [7, 8]
    ]
  ],
  "XX": [
    [1, 2],
    [3, 4]
  ],
}

This would result in terms like this:

• data["XYZ"][0, 0, 0] -> 1.0 * "X_0 Y_0 Z_0"
• data["XX"][0, 1] -> 2.0 * "X_0^2"

This is not a construct we currently have in Qiskit Nature, but it may prove to be useful in the future. In any case, supporting it in the PolynomialTensor is trivial.