Combined resolved/unresolved coupling methods showcase

Add a showcase for combining fully resolved and unresolved coupling methods for strongly heterogeneous particulate flows. For this to work some pe_coupling utility classes were modified to work with a body selector function. The related paper is attached here, but not referenced in the showcase yet as it was unclear to me what the convention is for walberla (may need some feedback whether i should reference the paper/author).bachelorarbeit.pdf