or more accurately: "use FIELD or METRIC units", LAB and PVT-M units
are still unsupported, but they seem to be pretty exotic and are also
not supported by opm-parser either...
note that this patch requires an API change (with the usual
consequences for all downstream modules which use this class) because
the deck needs to be passed to EclipseWriter. If somebody knows a
canonical way to get the names of the written units from EclipseState,
this is API change is not required.
instead, use Opm::EclipseState. This requires to pass the PhaseUsage
object to the EclipseWriter, as this one cannot be recovered from
EclipseState (yet?).
