Subsurface meshes via TetGen

This creates a subsurface representation (based on a 2D mesh + a set of raster layers) that can be used as input for creating a mesh with TetGen. The input mesh may include Tri and/or Quad elements, Line elements are removed as a preprocessing step.
Currently only a TetGen input file is written (*.smesh), then the mesh has to be created with TeGen and the resulting mesh files can be imported into OGS again.