replace ae_point_2d with a "point" class based on std::pair

src/aevol_modify.cpp

src/libaevol/Makefile.am

src/libaevol/ae_environment.cpp

src/libaevol/ae_environment.h

src/libaevol/ae_exp_manager_X11.cpp

src/libaevol/ae_fuzzy_set.cpp

src/libaevol/ae_fuzzy_set.h

src/libaevol/ae_fuzzy_set_X11.cpp

src/libaevol/ae_point_2d.h

src/libaevol/fuzzy.cpp

src/libaevol/fuzzy.h

src/libaevol/param_loader.cpp

src/libaevol/param_loader.h

src/libaevol/point.cpp

src/libaevol/point.h

src/post_treatments/ancstats.cpp

src/post_treatments/create_eps.cpp