Nice interface for space charge

[jank324]

Once space charge is implemented in #142, we should add a nice interface for it. There was discussion of adding a method Element.with_space_charge in #142.

[cr-xu]

I think apart from the spliting the element in fixed number of slices, spliting a segment in equal s (some maximal step size depending on the energy or sc strength?) should also be implemented, so that the SC kicks are homogenized in case of elements with very different lengths.

This would require however also an improved implementation of individual elements.