This works by having a "focus degree of freedom" during
linearization. When evaluating the local residual, all derivatives of
the residual/fluxes are with regard to the primary variables of that
DOF.
The two main offenders were the Forchheimer velocity model and the
model for the Stokes equations. To ensure that they continue to work,
the "powerinjection" and the "stokestest2c" problems are now both
compiled and tested with both, automatic differentiation and finite
differences, and the results of these tests is compared against the
same reference solution.
The majority of the time required to develop this patch was actually
required for testing: All tests compile and pass with debugging and
aggressive optimization flags with at least GCC 5, GCC 7 and clang
3.8, as well as Dune 2.3 and 2.4. Also, the results of flow_ebos stay
identical for Norne whilst the performance difference is below the
measurement noise on my machine. (the version with this patch applied
was actually about 1% faster.)
