XFEM: Remove triangles with zero (very small) area from element
subdivision.
XFEM: Prepared for crack growth (still much work remaining though).
XFEM: Started to implement XFemStatic: a solver that inherits from
NonlinearStatic and should allow simulations with propagating cracks.
XFEM: Added basic support for growing cracks: enrichment of new nodes
and updating of cut element partitions (state varibles in Gauss points
are not yet mapped to the new subdivision though).
XFEM: Added input record initialization for propagation laws.