[dune-functions] GridFunction interface

[Christian Engwer]

Hi Oli,

> I am not convinced either.  In fact I'd find 1) more unexpected than 2).
> If I have a function that depends on some coordinates I'd expect its
> derivatives to be wrt to these coordinates.  So if the function is a local
> function I'd expect the derivatives to be wrt to local coordinates.
> If I want them wrt global coordinates then I'd need to multiply by
> JacobianInverseTransform.

No, the problem was/is that you don't know what to do, because it
depends on your particular type of function. Or lets ask the other way
around, what would your local function return in case of the RT0
function. Would it already perform the piola transformation?

Christian