it is possible that a dune entity vanishes if its iterator gets out of
scope. Whether this is a problem or not seems to be be highly depend
on the used configuration...
also, clean them up a bit:
- do not use the intensive quantities cache directly anymore. (i.e.,
that code should now work if the IQ cache is disabled)
- do not fiddle with the global Jacobian matrix and residual vector
directly. Instead, implement the water fluxes to the reservoir as a
source term like wells.
one thing that did not become fully clear to me is if each aquifer
ought to be assumed to be in contact with the whole reservoir or just
a few cells on the boundary. The current implementation goes down the
former path, while, without any deeper knowledge, I would rather
suppose that the latter applies. maybe my understanding of this is
just too limited, though.
