moving constant functions out of src/stan/math/special_functions.hpp
removing include to math/special_functions.hpp
removed math/special_functions.hpp, removing references to math/special_functions.hpp
removed math/special_functions.hpp, removing references to math/special_functions.hpp
removed math/special_functions.hpp, removing references to math/special_functions.hpp
removed math/special_functions.hpp, removing references to math/special_functions.hpp