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.