Remove the diffOrder parameter completely

This patch completes the removal of the diffOrder template parameter. The design of this was broken from its very beginning. It played very badly with the virtual interface, and it is not known whether anybody really ever used it. Its removal has lead to a vast simplification of the dynamic interface.

Note that after this patch, there is no more way to find out which partial derivatives are implemented by the 'partial' method and which aren't. Carsten and I decided to not implement and interface for this before meeting the first person who actually needs this. Without such a person I for myself am not able to design a good interface.