pushed range check from Grid to partialDerivative so it could adjust its dx according to index difference after clamp.  changed conn_ull from a dim of symmats to a vec of them so it could inherit arithmetic operators and work out of the box with partialDerivative.  stored gamma_uu for some reason and stored conn_ull with the hopes of some day making a variadic template tensor class and having it generate covariant derivatives.