[staggered]fluxVars] Improve handling of Neumann BCs for the momentum eq
What this MR does / why does DuMux need it:
Improves the handling of Neumann BCs for the momentum eq. This should correctly account for the case when there is a Neumann bc for both the normal and tangential part of the stress.
Special notes for your reviewer:
Maybe we should add a test for Neumann BCs. Added some tests for Neumann BCs
Edited by Ned Coltman