Weighting kernel not getting used in `lineardiff`

[pavelkomarov]

__lineardiff__ doesn't use the weights generated in __slide_function__ https://github.com/florisvb/PyNumDiff/blob/b171ac5582732c703f24976e01651730a146f50c/pynumdiff/linear_model/_linear_model.py#L65C48-L65C69

I don't know whether this is intentional, but I suspect it's a bug. If we in fact don't need the kernel for lineardiff, we can remove that parameter.

[pavelkomarov]

In #93, I'm purely reworking the function signatures of this file to address #68. But I think if this is indeed a real bug, it came about due to some confusion between the user-facing functions, backing private helper functions, and the window-sliding function. There may be a more elegant way to do this where we don't have to manage 5 things, although I see the appeal of the current structure.

[pavelkomarov]

Okay, weights are getting used farther down in _slide_function and in fact don't need to be passed to the _lineardiff function itself. I'm going to close this one with the note that _linear_model.py needs a bit more attention per #69 so this kind of confusion is less likely.

Also, @florisvb points out _slide_function should maybe live in the utilities, which I think is a great idea.

All this and more after further improving the unit tests.