adding placeholder for more functions in stan/math/matrix.hpp, moving validate_vector out
moving validate_vector_nonzero_size out of stan/math/matrix.hpp
moving validate_vector_nonzero_size out of stan/math/matrix.hpp
moving validate_vector_nonzero_size out of stan/math/matrix.hpp
moving validate_vector_nonzero_size out of stan/math/matrix.hpp
moving validate_multiplicable out of stan/math/matrix.hpp
moving validate_matching_sizes out of stan/math/matrix.hpp
moving validate_matching_dims out of stan/math/matrix.hpp
moving validate_symmetric out of stan/math/matrix.hpp
moving validate_square out of stan/math/matrix.hpp
moving validate_square out of stan/math/matrix.hpp
moving validate_row_index out of stan/math/matrix.hpp
moving validate_row_index out of stan/math/matrix.hpp
moving validate_column_index out of stan/math/matrix.hpp
moving validate_column_index out of stan/math/matrix.hpp
moving validate_greater out of stan/math/matrix.hpp
moving validate_greater_or_equal out of stan/math/matrix.hpp
moving validate_less out of stan/math/matrix.hpp
moving validate_less_or_equal out of stan/math/matrix.hpp
moving validate_less_or_equal out of stan/math/matrix.hpp