Removes unnecessary GridType template parameter from Geometry.

While inspecting dune-cornerpoint I came across this third template
parameter which is used nowhere. It is not needed as explained in
a comment. This patch removes it from Geometry and updates the
rest of the module accordingly.
Remove redundant cartDims_ from CpGrid.

The same information is already available in logical_cartesian_size_.
cartDims_ was only used internally for computing IJK. Unfortunately,
it seems like we reading the grid from the legacy Sintef format cartDims_
never got initialized. Therefore this probably even fixes a real bug
in addition.
Merge pull request #52 from blattms/geometry_remove_gridimp_parameter

Removes unnecessary GridType template parameter from Geometry.
Merge pull request #53 from blattms/cpgrid_remove_cartDims

Remove redundant cartDims_ from CpGrid.