[dune-functions] New example for Taylor-Hood elements

[Christian Engwer]

Hi Carsten,

I didn't find time yet to answer in detail, but I'd answer the quick
thing directly ;-)

> I'm just trying to implement this. Unfortunately it will requires
> to make copies in the PowerNode. Does anyone have a better idea?

No, in PDELab we also store copies in the powernodes. Btw, you will
usually have the copies anyway, as you are using the TypeTree
infrastructure.

Christian