[Opm] Renaming dune-cornerpoint to opm-grid

[Atgeirr Rasmussen]

Dear OPM community!

We are considering renaming the dune-cornerpoint module to opm-grid.
After that, we'd like to migrate the grid functionality that currently resides
in opm-core to opm-grid (for example the UnstructuredGrid class and
the cornerpoint grid processing routines).

This is because:
 - all other modules start with "opm-" (and we don't expect the Dune project to maintain this module),
 - the module contains a non-cornerpoint grid (the PolyhedralGrid class).
 - collecting all grid-related functionality in one place is good.

The module will be usable as a dune module (as before).
There are some issues that must be resolved (module dependencies
between opm-core and opm-grid primarily), but that does not matter for
the initial step, which is just renaming.

If you have any comments or see any problems with this, please tell us,
and we'll have a discussion. Otherwise, I intend to do the renaming on
Thursday, January 28, and will send a separate message when it is done.
Any further change after that (moving of classes etc.) will be announced
later, when (if) it happens.

Atgeirr