[dune-functions] GridFunction interface

[Christian Engwer]

Hi Carsten,

> 1)Return derivatives wrt global coordinates. Then we must

this is what we once decided. We decided to always return
global-valued data. COming to the example with the vectors field, you
have might have somthing like an RT0 basis. In this case you compose
the solution by linear compination of normal-velocities. The result of
this function is local valued, unless you first map your local normals
onto global normals. Did this make it clearer?

Christian