Add exemplary codegen app.

Adds an exemplary application that uses the code generation pipeline. Extends the read me to see how to correctly specify the codegen CMake variables.

apps/example_app_codegen/CMakeLists.txt

+waLBerla_link_files_to_builddir( *.prm  *.py)
+
+waLBerla_generate_target_from_python(NAME LatticeModelGenerated FILE LatticeModel.py
+        OUT_FILES LatticeModel.cpp LatticeModel.h
+)
+
+
+waLBerla_add_executable ( NAME ExampleAppCodegen
+                          FILES ExampleApp.cpp
+                          DEPENDS blockforest core field lbm geometry timeloop gui
+                                  LatticeModelGenerated)
+