adding placeholders for tests
moving log_sum_exp out of src/stan/math/special_functions.hpp
moving log_sum_exp out of src/stan/math/special_functions.hpp
moving log_sum_exp out of src/stan/math/special_functions.hpp
moving log1p_exp out of src/stan/math/special_functions.hpp
moving log1p_exp out of src/stan/math/special_functions.hpp