Address executor and observable incompatibility

[bdg221]

Description

Fixes #2449

This PR address code changes in the Executor and Pauli classes to account for when an Observable is passed to an executor along with a circuit that already has a measurement on a qubit effected by the Observable.

Also, I removed a check in the Executor class, with the assumption the user is passing a valid executor if an observable is used.

Furthermore, the docs for the Executor and Observable have been updated with the annotation/typehinting requirement.

Finally, additional unit testing has been added to ensure that incompatible Executors and Observables are caught and an appropriate message is given to the user.

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License

• [X] I license this contribution under the terms of the GNU GPL, version 3 and grant Unitary Fund the right to provide additional permissions as described in section 7 of the GNU GPL, version 3.

Before opening the PR, please ensure you have completed the following where appropriate.

• [ ] I added unit tests for new code.
• [X] I used type hints in function signatures.
• [X] I used Google-style docstrings for functions.
• [X] I updated the documentation where relevant.