Add new parameter fitter

[ma-sadeghi]

Closes #84

This PR adds a new parameter fitter. It still uses the same method as impedance.py, but is 20x ~10x faster, so we can afford to brute-force our way into a pottentially good fit.

It also standardizes the circuit function signature: Used to be fn(p, freq), now is fn(freq, p) to be intuitive. Also all functions that accept Z and freq, now accept it in the following order: freq, Z, again to be intuitive.