Added a 'cap' type of smoothing to WeakPlaneShear

Added two tests demonstrating the idea and its correctness

Also modified MohrCoulomb and WeakPlaneTensile to remove unused variables

Addresses #4125 .
Note that i intend to do the same 'cap' stuff in MohrCoulomb and Tensile at some stage soon.
Merge pull request #4126 from WilkAndy/tip_4125

Added a 'cap' type of smoothing to WeakPlaneShear