Added warning when trying to use high order Lagrange elements

When using Lagrange elements, only order 1 and 2 are supported. Furthermore, order 2 requires the mesh to be itself second order (I think). This PR adds a warning when trying to use higher order and recommends using hierarchical H1 basis instead. I guess a similar warning should be used for all function spaces where the order isn't arbitrary, but I'm not confident enough to be sure.