Replaced GroverOperator usage by grover_operator

[tnemoz]

Summary

Fixes #76 and fixes #241. The AmplificationProblem and EstimationProblem classes now use the grover_operator function instead of the deprecated GroverOperator class to build their respective oracle. The tests have been adapted to test for both, and the tutorial uses the new function.

The access to attributes of GroverOperator has been replaced with that of AmplificationProblem.

[coveralls]

Pull Request Test Coverage Report for Build 17765604336

Details

• 9 of 11 (81.82%) changed or added relevant lines in 3 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage decreased (-0.02%) to 90.176%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
qiskit_algorithms/amplitude_amplifiers/amplification_problem.py	4	5	80.0%
qiskit_algorithms/amplitude_estimators/estimation_problem.py	4	5	80.0%

Totals
Change from base Build 17759027522:	-0.02%
Covered Lines:	6664
Relevant Lines:	7390

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💛 - Coveralls

[Cryoris]

The grover_operator function has only been introduced in Qiskit v1.3, but the qiskit-algorithms repo is currently stated to be compatible with qiskit>=1.0. To avoid a breaking change and remain compatible with Qiskit 1.0 we would have to treat both GroverOperator and grover_operator.

[Cryoris]

We could use qiskit.__version__ to check whether to return GroverOperator or grover_operator here. This should also be documented then

[tnemoz]

In order not to introduce a breaking change then, is it OK if the function uses GroverOperator if the Qiskit version is 2.1.2 or older, and otherwise uses grover_operator? That way, it would prevent a breaking change from users upgrading qiskit-algorithms without upgrading qiskit.

This of course assume that the next qiskit-algorithms release is done before Qiskit 2.2.0 is out! Or we could test for < 3.0, since this is when GroverOperator is removed if I'm not mistaken. What are your views on this?

[Cryoris]

Good questions -- my preference would be to use the grover_operator function whenever possible (i.e. qiskit>=1.3) since it's more efficient than the GroverOperator. But maybe it's safest to just sell this as bugfix and start using grover_operator with qiskit>=2.1 since the GroverOperator was deprecated in 2.1.

[tnemoz]

I went for the latter and thus updated the release note to categorize this as a bug fix. Does that work for you?