[staggered] Improve Beavers-Joseph slip condition

Kilian Weishauptrequested to merge
feature/improve-beavers-joseph into master

closes #713 (closed)

So far ths BJS condition only considers du/dy. This MR extends this to (du/dy + dv/dx)

Furthermore, a velocity in the PM can now be specified.

Edited by Ned Coltman

Merge request reports