Add PolynomialTensor class

[SaashaJoshi]

Summary

Adding a [draft] file for PolynomialTensor class.
[WIP] corresponding unittest.
Closes #665

Details and comments

The PolynomialTensor class overrides:

1. _mul and _add functions from StarAlgebraMixin LinearMixin
2. conjugate and transpose methods from AdjointMixin

More brainstorming needs to be done for compose and adjoint functions.

[CLAassistant]

All committers have signed the CLA.

[mrossinek]

Some suggestions from my side. Other than that, this looks to be shaping up quite nicely 👍

[coveralls]

Pull Request Test Coverage Report for Build 3052969424

• 65 of 80 (81.25%) changed or added relevant lines in 2 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage decreased (-0.02%) to 85.299%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
qiskit_nature/second_q/operators/polynomial_tensor.py	64	79	81.01%

Totals
Change from base Build 3051660763:	-0.02%
Covered Lines:	16809
Relevant Lines:	19706

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💛 - Coveralls

[mrossinek]

I had another discussion with @woodsp-ibm and we said that it might be beneficial to directly implement PolynomialTensor as a subclass of Mapping. That would give full flexibility when accessing the internal _data object.
The way that you could achieve this, is by something similar to the following:

from collections.abc import Mapping
from typing import Iterator

class PolynomialTensor(..., Mapping):
    ...

    def __getitem__(self, __k: str) -> np.ndarray | Number:
        return self._data.__getitem__(__k)

    def __len__(self) -> int:
        return self._data.__len__()

    def __iter__(self) -> Iterator[str]:
        return self._data.__iter__()

This will directly enable users to use len(...), .keys(), .values(), .items() and even use p_t["key"] to access specific entries 👍

[mrossinek]

Please also double check the formatting of the docstrings. Especially PolynomialTensor should always be written as a single word

[mrossinek]

Just some minor comments. Other than this, I think this is almost good to go 👍

[mrossinek]

I'm happy with this as is! Thanks a lot, Saasha, for your continuous effort on this! 🙂

I'm sure that there will be some minor changes that I will be doing to this as I work on #666 but overall this looks like it is in very good shape to be merged now 👍

[mrossinek]

For a future reviewer: I will flesh out this docstring in an upcoming PR when working on #666 👍