Use default bounding box for `FullGridCellList`

As suggested in https://github.com/trixi-framework/TrixiParticles.jl/pull/660 by @svchb, it would be more convenient if the minimum and maximum coordinates of the semidiscretization would automatically be used for the bounding box.

Here, we want to create a cell list template without a bounding box, and the semidiscretization should set the bounding box when copying the NHS.

cell_list = TrixiParticles.PointNeighbors.FullGridCellList()
semi = Semidiscretization(fluid_system, boundary_system,
                          neighborhood_search=GridNeighborhoodSearch{2}(; cell_list))

Here, we want to use a larger bounding box, so the semidiscretization should not overwrite an existing bounding box of the cell list.

cell_list = TrixiParticles.PointNeighbors.FullGridCellList(; min_corner, max_corner)
semi = Semidiscretization(fluid_system, boundary_system,
                          neighborhood_search=GridNeighborhoodSearch{2}(; cell_list))

That would make the copy_neighborhood_search API awkward.

copy_neighborhood_search(min_corner=..., max_corner=...)

would only use min_corner and max_corner to set the bounding box when there is none in the template.
When the cell list already has a bounding box, these arguments would have to be ignored, or the second snippet above would not work. This sounds like a bad API to me.

Trace FullGridCellList construction through GridNeighborhoodSearch, Semidiscretization, and copy_neighborhood_search. Confirm where the semidiscretization copies the neighborhood search; done means an absent cell-list bounding box uses the semidiscretization coordinates while an explicitly supplied bounding box remains unchanged, without adding copy_neighborhood_search arguments.