No data replacement for mesh layer mapping

This introduces several changes when mapping meshes based on rasters:
* for 2D meshes a "default" value can be set for nodes on NoData-pixels
* this "default"-value is also used when no raster info is available (i.e. the raster extent is smaller than the the mesh extent
* for 2D meshes the option to remove mesh nodes at NoData values has been removed (was never used anyway)
* for 3D meshes with constant thickness layers are ignored if they have thickness=0 (i.e. these layers are not created)
* for 3D meshes based on rasters a node with no information (i.e. NoData or outside of raster) will get the elevation of the node right above it, for the top-most layer the default value is "0"

The new functionality creates an invalid mesh if a 3D mesh is created based on rasters where the top most layer does not cover the whole mesh-extent and layers below have an elevation > 0. If anyone has an idea how to easily avoid this, pls let me know.
Also, this does *not* fix the problem of invalid elements/meshes in case of intersecting subsurface layers.

This PR especially allows to create subsurface meshes with incomplete subsurface layers is node collapsing is applied after mesh creation.