Update GaussianPulse for consistent parameter validation results whether validation occurs during or after construction.

[upsideon]

Summary

Fixes #7882 by moving a check for the simultaneously specification of width and risefall_sigma_ratio from qiskit.pulse.library.GaussianPulse.validate_parameters to the class constructor. This ensures that when the construction process internally sets risefall_sigma_ratio using the value of width, that subsequent validations, via calls to validate_parameters do not fail erroneously.

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[coveralls]

Pull Request Test Coverage Report for Build 2488093915

• 2 of 2 (100.0%) changed or added relevant lines in 1 file are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage remained the same at 84.424%

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Totals
Change from base Build 2484981305:	0.0%
Covered Lines:	54712
Relevant Lines:	64806

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💛 - Coveralls

[nkanazawa1989]

Probably extra documentation is confusing since this is a public method. I prefer moving duplication check of (width, risefall_sigma_ratio) from .validate_parameters method to the GaussianSquare pulse constructor.

Same modification has been made in GaussianSquare of the SymbolicPulse (which is replacement of ParametricPulse). Details in #7821 (symbolic_pulses.py)

[upsideon]

Thank you for the review, @nkanazawa1989! That is a good approach. I've applied the required code changes and added a test that would have caught this bug and will catch others like it, if they occur.

[nkanazawa1989]

Thanks, this is nice bug fix :)