Make parameter_values optional

[rathishcholarajan]

Summary

• Make parameter_values optional so we don't have to do [[]] * n
• Allow passing just one circuit, ex: circuits=bell instead of circuits=[bell]

Details and comments

Fixes #

[jyu00]

While we are at this, we can make the __init__() functions match the implementation as well, e.g.

class Estimator(BaseEstimator):
    """Estimator primitive."""
    def __init__(
        self,
        circuits: Union[QuantumCircuit, Iterable[QuantumCircuit]], 
        ...
    ):

This allows one to do estimator(circuits=bell) instead of estimator(circuits=[bell]) (the former is also closer to Qiskit convention)

[coveralls]

Pull Request Test Coverage Report for Build 1981646463

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Details

• 4 of 4 (100.0%) changed or added relevant lines in 4 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+1.2%) to 70.158%

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Totals
Change from base Build 1971188683:	1.2%
Covered Lines:	2043
Relevant Lines:	2912

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💛 - Coveralls