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 #53 from blattms/cpgrid_remove_cartDims

Remove redundant cartDims_ from CpGrid.