[dune-pdelab] Interface changes for containers in LocalOperator

[Christian Engwer]

Hi Jö,

> > access to vector: x(lfsu,i)
> > access to matrix: jac(lfsv,i,lfsu,j)

> What does this do?  Is this ro access to the unweighted container entries?  Or
> rw?

The x is already a const reference, so this method is for read-only
access. The main idea is that used should not have to deal with two
different indices. What ever is computed in the local operator is
working with an index of the size of the current lfs. The containers
are working with indices of the size of the root-lfs. This new methods
hides this fact and allows to access  the same container with the
indices of the local function space.

Christian