Add support for inactive particles

[LasNikas]

The open boundaries in trixi-framework/TrixiParticles.jl#442 require inactive and active particles.

So far, we avoid adding inactive particles to the neighbor lists by setting very large coordinates.
This could be circumvented by passing active_coordinates instead of current_coordinates.

The problem is, that we then have to remove inactive particles from the neighbor lists in every update_nhs!.