added parameter to allow user to specifiy minimum contact area for fractal model
added missing virtual declarations
A new plasticity model for geomaterials is added.
added implementation of AddNodalForce
1. committing changes
commented out #endif comment
added missing namespace Tahoe declarations
added missing namespace Tahoe declaration
added Majumdar-Bhushan contact model
Merge of voight code and bug fixes (no more nan)
