Add parameters property function to BaseCircuit class

[adrianusler]

In issue #285, a user was looking for a way to obtain the circuit model parameters as an array. The suggested workaround of accessing the parameters_ attribute directly is not very elegant since it is a private parameter. To circumvent this, I have added a property routine to the BaseCircuit class, so one can now access the model parameters with

parameters = <circuit object>.parameters

I would argue that this closes #285.

[coveralls]

Pull Request Test Coverage Report for Build 10994619609

Details

• 3 of 3 (100.0%) changed or added relevant lines in 1 file are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage remained the same at 99.883%

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Totals
Change from base Build 10823700912:	0.0%
Covered Lines:	857
Relevant Lines:	858

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💛 - Coveralls

[adrianusler]

I believe this is ready to merge, @mdmurbach.