adding placeholders for functions in stan/agrad.hpp
moving exp out of stan/agrad.hpp
moving log out of stan/agrad.hpp
moving log10 out of stan/agrad.hpp
moving sqrt out of stan/agrad.hpp
moving pow out of stan/agrad.hpp
moving cos out of stan/agrad.hpp
moving sin out of stan/agrad.hpp
moving tan out of stan/agrad.hpp
moving acos out of stan/agrad.hpp
moving asin out of stan/agrad.hpp
moving atan out of stan/agrad.hpp
moving atan2 out of stan/agrad.hpp
moving cosh out of stan/agrad.hpp
moving asinh out of stan/agrad.hpp
moving tanh out of stan/agrad.hpp
moving tanh out of stan/agrad.hpp
moving floor out of stan/agrad.hpp
moving floor out of stan/agrad.hpp
moving fmod out of stan/agrad.hpp