PhaseOracle and BooleanExpression allow setting the expression variable order

[boschmitt]

Summary

This PR add the ability for users to define the variable order when defining a
PhaseOracle. This closes #6499. It depends on #6588.

from qiskit import *
from qiskit.tools.visualization import plot_histogram
from qiskit.circuit.library import PhaseOracle
from qiskit.algorithms import Grover, AmplificationProblem

oracle = PhaseOracle('((A & C) | (B & D)) & ~(C & D)', var_order=['A', 'B', 'C', 'D'])
problem = AmplificationProblem(oracle=oracle, is_good_state=oracle.evaluate_bitstring)
backend = Aer.get_backend('qasm_simulator')
grover = Grover(quantum_instance=backend)
result = grover.amplify(problem)
print(result.circuit_results[0])

The result is {'0101': 237, '1010': 271, '1011': 276, '0111': 240}

Details and comments

The default behaviour is to order variables by appearance in the expression.

Which prints the result: {'0011': 266, '1101': 264, '1100': 241, '0111': 253}

[1ucian0]

On hold, as it depends on #6588.

[1ucian0]

#6588 is on its way to be merged. So removing on hold

[1ucian0]

Do you mind adding some testing and reno?

[CLAassistant]

All committers have signed the CLA.

[coveralls]

Pull Request Test Coverage Report for Build 4087465935

• 3 of 3 (100.0%) changed or added relevant lines in 2 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage remained the same at 85.245%

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Totals
Change from base Build 4083955049:	0.0%
Covered Lines:	67109
Relevant Lines:	78725

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💛 - Coveralls

[1ucian0]

thanks!