Provide dynamic information of derivatives implemented by `partial()'
When we needed the number of implemented partial derivatives statically for
evaluate() this was encoded in the enum
diffOrder. Since the static interface of the new
partial() method is independent of this number, we should provide this information dynamically. To this end I propose to introduce the method
bool hasPartial(usigned order) const
returning true if all partial derivatives up to the given order are implemented. By not returning the max-order itself, this allows to implement an arbitrary number. If we want to provide refined information in case only some derivatives are implemented, we may also provide
bool hasPartial(const std::array< unsigned int, dim> &order) const
to check for an individual partial derivative. While the latter alone would already provide all information it's cumbersome to use. Hence this is only a possible add-on in case someone has a use case (I'm currently not aware of one).