Makes distinction between functions more clear. (#176)

Currently, there are two abstract interface for the grids. One that
usually returns pods and arrays of them that also can be used by C
and is used also in opm-core, and one that returns Eigen datastructures
 needed within opm-autodiff.

This commit adds a postfix ToEigen to those functions (faceCells, and
cellCentroidsZ) one could imagine to also return pods and arrays of them.
This should at least resolve the confusion about the two faceCells functions.

The next step will be issue #192
Fixes #176