> Is that a fully formed (compressed) grid or some intermediate structure that includes the complete corner-point description in an accessible way?

I had a brief look at it and it is not a grid at all, so it is possibly a bit of a misnomer. Rather, it seems to be the c++ equivalent of opm-core's `struct grdecl`. (it also includes the code to construct the ZCORN, ACTNUM, MAPAXES and COORD data regardless of how the grid is specified  in the input deck, which is a thing I _really_ like :).)