Added access methods needed for fully implicite blackoil in opm-autodiff.

Namely:
- Access to begin iterator over the cells and faces.
- Access to the faces of a cell.
[CpGrid] Make the objects return by begin{Face,Cell}Centroid iterator over centroids.

This in line with the last changes to opm/core/refactor-for-cpgrid. The iterator returnd
now has the centroid (aka FieldVector<double,3>) as value type.
removed superfluous const from value type
Added function to query the number of faces.
Merge pull request #72 from blattms/refactor-for-cpgrid-support

Refactor for cpgrid support