Use the Opm::EclipseGrid class instead of the deck directly

this has the advantage that the gazillion ways of specifying an ECL
grid are abstracted away, and thus e.g. using D{X,Y,Z}V is
automatically supported. Also, if additional grid construction methods
are implemented in Opm::EclipseGrid, these will automatically be
supported by dune-cornerpoint as well. (If the API for
Opm::EclipseGrid does not change, that is.)