Generalize Slip Condition

What this MR does / why does DuMux need it:

• All slip condition related functions should not be in base problem but instead implemented on level of test problem
• One should be able to switch slip conditions. Currently BJ was always assumed.

Notes for the reviewer

This is a first draft. There are still some things we should discuss.

TODOs:

• Can we unify the slip velocity interface? The issue with fcstaggered currently is the fact that the user can pass tangentialVelocityGradient which allows to overwrite derivatives
• Is the current BJ implementation even correct in 3D? I would have expected that it is not enough to only consider one single tangential direction
• Also introduce BJS