Alias BaseOperator.__mul__ to BaseOperator._multiply in quantum_info

[daimurat]

Summary

Introduced a left side scalar multiplication to Operator class in quantum_info (issue: #8006 )

Details and comments

Add __mul__ method to Operator class to enable scalar multiplication not only from right side but left side.
It can be used by using * decorator,

For example

from qiskit.quantum_info import Pauli
Pauli("X") * -1j

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[coveralls]

Pull Request Test Coverage Report for Build 2291373536

• 2 of 2 (100.0%) changed or added relevant lines in 1 file are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+0.0005%) to 84.358%

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Totals
Change from base Build 2272306729:	0.0005%
Covered Lines:	54487
Relevant Lines:	64590

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💛 - Coveralls

[ikkoham]

Thank you very much. I think it will be useful. I have a minor comment

[jakelishman]

Looks good to me, thanks for this! I left a small amount of historical context in #8006 (comment) as a note to us in the future, but from my side this looks good to merge. Ikko will be able to tag this "automerge" and it'll get merged.

[ikkoham]

LGTM! Thank you.