Add ElementaryOpoerator.define override option

[ikkoham]

Description

There are cases where we want to experiment with various definitions of the Hamiltonian and collapse operators.
Of course, it's possible to modify the id during execution or exploit lazy evaluation (I will mention this later), but these approaches don’t feel intuitive.
Therefore, I'd like to propose an option to allow overrides. By using this, users will be able to conduct experiments with different definitions more intuitively and effectively. Especially when developing with Jupyter Notebook, it’s quite inconvenient to have to restart the kernel every time we want to change a definition.

This proposal allows

ElementaryOperator.define(
    "operator",
    expected_dimensions=[n],
    create= lambda: operator1,
)
ElementaryOperator.define(
    "operator",
    expected_dimensions=[n],
    create= lambda: operator2,
    override=True,
)

Alternatives (Workaround)

Use different id.

ElementaryOperator.define(
    "operator1",
    expected_dimensions=[n],
    create= lambda: operator1,
)
ElementaryOperator.define(
    "operator2",
    expected_dimensions=[n],
    create= lambda: operator2,
)

Redefine the operator itself. (lazy evaluation)

ElementaryOperator.define(
    "operator",
    expected_dimensions=[n],
    create= lambda: operator,
)
operator = operator2

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