for the gas-only case with vaporized oil enabled, the oil pressure was
used, but ebos uses the gas pressure as a primary variable in that
case. (the reason is that it is faster to check if oil appears if the
gas pressure is available in that case.) This patch corrects that and
does no longer use the PrimaryVariables::set$FOO_PV() methods because
these methods are hard to keep in sync with the indices for the
primary-variables-as-a-Dune::FieldVector feature (which is actually
quite important for the linear solver).
it uses ebos for linearization of the mass balance equations and the
current flow code from opm-simulators for all the rest. currently, the
results match the ones from plain `flow` for SPE1, SPE9 and Norne, but
performance is not optimal: on SPE9, converting from and to the legacy
data structures takes about a third of the time to do the actual mass
balance assembly. nevertheless `flow_ebos` is almost as fast as plain
`flow` for SPE9. (for Norne `flow_ebos` is about 15% slower, even
though the results match quite closely. the reason for this is that it
requires more iterations for some reason.)
