Allow float input to BaseCircuit.predict function

[adrianusler]

I have made a small change to the BaseCircuit.predict function that now allows both floats and lists/arrays are inputs.

frequencies = (
    np.array([frequencies], dtype=float) if isinstance(frequencies, float)
    else np.array(frequencies, dtype=float)
)

So now, one can predict impedances at a single frequency (say, 10.) by calling

Z = <circuit object>.predict(10.)

Closes #277.

[coveralls]

Pull Request Test Coverage Report for Build 10994591777

Details

• 1 of 1 (100.0%) changed or added relevant line 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:	854
Relevant Lines:	855

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