Add a prependHyTeGMeshDir() function
As discussed internally we would like to make opening "internal" mesh files in tests and tutorials resilient against re-location of source code files.
This MR suggest a solution by adding a HYTEG_MESH_DIR macro that stores the absolute path to "data/mesh" inside the build directory. Additionally it adds a free function prependHyTeGMeshDir() that can be used to prepend this path to a filename or relative path.