It is not necessary anymore

As a step towards making the driver the main function.

In ULF the jump is defined as jump(v) = v('+') - v('-'). Accordingly
the outer unit normal has positve restriction and ('+') refers to self
und ('-') to neighbor. Unfortunatly you can also find '+' and '-'
swapped in the literature.

The control operator doesn't accumulate in the residual. Instead the
localoperator has a member vector where the value of the derivative is
accumulated.

In that case you need two GridFunctions, which can be
bound to the two cells.

After consulting Steffen