[dune-functions] Range type for scalar grids functions

[Christian Engwer]

Dear all,

while checking that my pdelab implementation of a
discretegridviewfunction adheres to the dune-functions interface I
encountered something, which might be well known to you ;-)

I was deriving the Range type from LocalBasis::Traits::RangeType. This
means that for scalar functions our return type is
Dune::FieldVector<RF,1> and the DerivativeRange is consequently a
FieldMatrix.

Now this is surprising for the user (and I did the same mistake in my
tests), as the user would assume that the scalar function return just
RF. How do you interpret the situation? Should we live with this
situation and inherit the return type form localfunctions, or should
be work around this and force scalar functions to return RF? We could
easily achieve this by yet an other traits class ;-)

Ciao
Christian