Added first example

src/CMakeLists.txt

src/example_2p_cahnhilliard/2p_cahnhilliard_equation.cc

src/example_2p_cahnhilliard/2p_driver.hh

src/example_2p_cahnhilliard/CMakeLists.txt

src/example_2p_cahnhilliard/chns.ini

src/example_2p_cahnhilliard/grids/periodic.dgf

src/example_2p_cahnhilliard/grids/xperiodic.dgf