[multidomain,localoperator] Further cleanup and restructure of the localoperators.

This time:
  - deprecated old coupling12() and coupling21() and replaced them by one coupling()
  - try to group boundary condition types
  - unified naming and steps how fluxes are calculated (nothing new was added)
  - further replacing _s, _n by 1, 2

boundaryLayerModel: spurious changes in (~4% for 1e-11 and 1% for 1e-9 in velocity)