add Distribution function
update coryright year
testing spatial function does not need to be combined with mesh nodes. make it simple
add tests for generating distributed nodal values
adjusted LayeredVolume-interface to take raster files from bottom to top
changed static methods in MeshLayerMapper to non-static
slightly changed creation of static subsurface layers
moved raster i/o to FileIO/AsciiRasterInterface
moved raster interpolation and raster extend test to raster class
reworked prims-layer-generation to work from top to bottom and prefer older (lower) layers in cases of conflicting information
simplyfied layerMapping method to only handle single layered meshes
adjusted layer mapping dialogue
added a base class to MeshLayerMapper and LayeredVolume for handling basic functionality and class structure
formatting issues
removed static keyword in friend def
changes based on pr comments
std namespaces
Added tetgen 1.5.0 as a submodule.

This module is automatically checked out when OGS_BUILD_GUI=ON and the executable
gets packaged with the Data Explorer.

Tetgen source is hosted at https://github.com/ufz/tetgen.git
Use a relative path to precompiled VTK.

Related to #469.
added missing word to comment