Include information about simulation domain boundaries outside of mesh records

[AngelFP]

For data visualization purposes, it is useful to have information about the extent of the simulation domain of a PIC simulation even when only particle species are present in the output. Currently, however, one can only get this information if the output contains at least one field (mesh record).

As an example, this information would allow VisualPIC to set the spatial limits of a 3D render of a particle species to the extent of the PIC simulation box, so that the extent of the render does not fluctuate between time steps.

In a recent meeting, @ax3l suggested that the particlePatches sub-group could be used for this purpose, but this approach might not work if some particles eventually leave the simulation box, as it can happen in some simulation codes.