add the PORV grid property and use NaNs as default values for most double grid property

In grid properties NaNs mean "I can't compute that value", either
because the parser needs information it cannot have (e.g. the position
of the cell centers for the endpoint scaling keywords which are
initialized using the ENPTVD tables or because the deck did not
specify a mandatory value like PERMX)
check for the IMPTVD keyword for the I* grid properties

the effect is quite limited: usually it uses NaNs if the IMPTVD/ENPTVD
initialization route is taken. the only difference is if there is
IMPTVD but no ENPTVD (or vice versa), but I somehow doubt that this is
a common case...