Circuit Element Unit scalable feature

[AvMedic]

I found that the circuit.fit() will call scipy.opitimize for fitting purpose and the step size is 1e-10 limited by default.
Since impedance.py uses F and H as unit, a high frequency range problem will easily go to pF and nH level, and the step size will be a problem for fitting.
[Example]My example is a RLC circuit resonant point at around 8MHz.
circuit = 'p(R1-C1-L1,C0)' initial_guess = [4.56, 80e-12, 4.83e-6, 780e-12]
I actually built an accurate csv data file based on initial_guess for a try and ensured this issue. The solver will revise 80pF capacitor to 1e-10F and fit a bad curve.
[Fitting Options] I tried to revise fitting options, however I see that some methods don't support adjusting step size, some support but don't support complex number, and some options don't work. (I'm no expert in fitting functions.)
[Workaround] I directly revised elements.py to pF and pH. And good fitting result shows.
[Potential Enhancement] Add unit options for varias usercase.

[adrianusler]

I also had this problem and submitted an issue to the scipy repo: scipy/scipy#18793 (comment)
I think this problem with scipy has been solved by now, so upgrading scipy should fix that problem.