Fix location of VTKmCheckPyexpander.cmake
The location of VTKmCheckPyexpander.cmake was originally set to $ {CMAKE_SOURCE_DIR}/CMake. This is correct with respect to the VTK-m install, but incorrect if VTK-m is being included as a module in another project (like VTK). Change the location to ${VTKm_CMAKE_MODULE_PATH}, which should be correct in every case.