Dead Kernel when passing an empty array to get_subset function

[hurricane642]

Hello everyone! Attempting to use get_subset function for rascal.neighbourlist.structure_manager.AtomsList with argument equal to [] causes Dead Kernel. Need to check the input data for this function

[max-veit]

The segmentation fault probably comes from #400 (or something closely related) after get_subset(), which does no bounds checking, passes through an empty selection.