[dune-functions] GridFunction interface

[Christian Engwer]

Hi Carsten,

> B)One reason to return LocalFunction as object, is that they
>   should behave like any other function. However this is
>   not exactly true for its derivative, because the returned
>   derivatives are computed wrt. global coordinates for local
>   coordinate arguments.
> 
>   For me this is not a real problem. But we must clearly state
>   this in the docs.

this is not exactly true. The same problem already occures for the
value, if you have a vector valued Range. If your Range describes a
velocity field, or similar, you have to transform the vector to map
from local valued data to global valued data.

Christian