[cleanup] Get rid of 'evaluate()' in virtual interface

Since evaluate(...) for partial derivatives is gone, we can clean up the virtual interface considerably. Now we have a single interface layer instead of a hierarchy with one layer per diffOrder.

This also drops the LowerOrderLocalBasisTraits, because they where only needed as implementation details of the implementation and the wrapper.

Since diffOrder is now completely useless (but still present in LocalBasisTraits), we canonicalize its value to 0 using a template alias. As a consequence LocalFiniteElementVirtualInterface<LB> is now exactly the same interface regardless of the diffOrder value in LB.

Without this trick you may run into trouble because you have different interfaces depending on the value of diffOrder.