Element Slicing: Strang Splitting

[ax3l]

@RemiLehe suggested today we could use a weighted approach to ds slices (Strang Splitting) when we slice our elements (for collective effects), to increase the numerical order by one.

I think we could even use a generator for the slices we track in an element, to implement various splitting methods.

cc @cemitch99

Example:
https://github.com/desy-ml/cheetah/blob/v0.7.0/tests/test_space_charge_kick.py#L46-L64