add CMake example using add_subdirectory

CMakeLists.txt

example/CMakeProjectSubdir/ALL_Staggered.cpp

example/CMakeProjectSubdir/ALL_Staggered_f.F90

example/CMakeProjectSubdir/CMakeLists.txt

example/CMakeProjectSubdir/all_contains_source_tree

example/CMakeProjectSubdir/build_all.sh

src/CMakeLists.txt