casts frequency and impedance inputs to np.array

[mdmurbach]

Fixes #206 #208 to allow more flexibility in how inputs are passed into fitting, prediction, plotting functions

[coveralls]

Pull Request Test Coverage Report for Build 3170238825

• 23 of 23 (100.0%) changed or added relevant lines in 3 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage remained the same at 100.0%

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Totals
Change from base Build 3136829995:	0.0%
Covered Lines:	841
Relevant Lines:	841

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💛 - Coveralls