XFEM: Started to couple cohesive zones with XFEM.
XFEM: Coupled cohesive zones with XFEM. Added bilinear cohesive zone
model.
XFEM: Added test case for cohesive zone coupled with XFEM.
XFEM: Added possibility to use both analytical and numerical tangent for
the cohesive zone contribution.
Fixed conflict in xfemstatic.C
Fixed some warnings and cleaned up.