this basically comes down to adding a few flag keywords but also
requires to add a few E300 fields to TABDIMS...
the grid cannot be instantiated yet as these deck seem to use some
peculiar constructs to specify the grid data. In particular it uses
property modifiers for keywords that specify the grid. (which might be
not allowed by Eclipse but work anyway...) the deck contains something
like
```
GRID
EQUALS
DX 50.0 /
DY 50.0 /
DZ 50.0 /
/
```
which Opm::EclipseGrid can't handle yet because grid properties are only
evaluated after the grid has been instantiated...
c73cb0b39a