Vector Fitting: Custom pole initialization

[Vinc0110]

As requested in the mailing list, this PR adds an option for custom pole spacing by the user.

The pole spacing is a delicate part of the fitting process and this option is supposed to be used by advanced users only.

The goal was to keep the API unchanged, and I hope this is an elegant and usable way to do it.

[andree-sc]

Hey, thanks a lot, looks fine, except for one small thing I noticed: in line 187ff. I think a check should be made whether or not the instance variable "self.poles" holds any poles... I don't think it's that relevant, but it's the only thing I noticed now.
Other than that, nice work. I think it's an elegant solution and it doesn't change anything about how the API works thumbsup

[jhillairet]

OK. Would like it to be included in the next release ?

[Vinc0110]

Yes! I'm currently working on the last few tweaks. Almost done...

[Vinc0110]

I don't understand why the tests are failing.
@FranzForstmayr Any idea? Is it because of that decorator to filter warnings?

https://docs.pytest.org/en/6.2.x/warnings.html#pytest-mark-filterwarnings

[Vinc0110]

Ok now. I moved the tests to a separate PR (#850). This one is ready.

[Vinc0110]

@andree-sc: Thanks for the review! I added the parameter check.