BeamMonitor: Enable Filtering by Cycle (Turn)

[ax3l]

For modeling of periodic lattices, we can now sub-select the output of particles at the BeamMonitor/beam_monitor element to every N-th period (turn or cycle).

This is first, simple (equidistant) implementation of #662.

To Do

• [x] implement
• [x] document
• [x] add example
• [x] finalize user-facing naming of filter

[ax3l]

@cemitch99 this is ready for review & merge now :)