[navierstokes][problem] Do not expose discretization methods in problem

This created some unresolved dependenices due to the cyclic interdependency
of problem and coupling manager. This worked before because the discretizaiton
method was always one specific type. With tags the actual type has to be known,
so both problems and the coupling manager have to be instatiated before this
information can be accessed.
5 jobs for !2870 with feature/remove-navierstokes-problem-discmethod
latest detached
Status Name Job ID Coverage
  Trigger
passed full-dune-latest-release-clang

passed full-dune-latest-release-gcc

passed full-dune-master-gcc

passed minimal-dune-latest-release-gcc

 
  Downstream Modules
passed trigger lecture