separate topology traits data and accessor functions

Original intent was to get rid of clang warnings like this:

dune/dune-alugrid/dune/alugrid/3d/topology.hh:240:12: warning: instantiation of variable 'Dune::ElementTopologyMappingDune::tetra::generic2aluFace_' required here, but no definition is available [-Wundefined-var-template]

These occured because the definitions were provided in a separate translation unit 3d/topology.cc and not known at time of instantiation. The patch moves traits data to a seperate templated traits class and lets the mapping class inherit from it. Better readability is a bonus.