Monitor Helper: Add every ds & Slice Elements

[ax3l]

Phase space animation: we need a convenient way to add a monitor every `ds

• in an s interval between [a;b], dump N times equally spaced in ds
• For general animation of the beam evolution.

Implementation ideas:

• Add a helper function to add monitors into a lattice with inputs given above.
• Details: sort and add thin monitor elements.
• Details: find if the requested position slices an element. Slice it up into multiple elements if hit.
• We should even be able to implement this in C++, not only Python.