This is currently only used for level 4 instrumentation,
as these start and stop markers were messing heavily with
dependencies in the sum factorization code. This way, the necessary but
ugly work of adding instrumentation is separated much more
nicely.

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