Added free method to get the sum of the the faces that are attached to every cell.
Added method to access face normals.
Added traits classes for the type of cell and face centroids.
Added documentation for FaceCellTraits
Made trans_tpfa usable with grids apart from UnstructuredGrid.
Fixed unused parameter warning in initBlackoilStateFromDeck.
Added missing namespace qualifiers.
Added support for computePoreVolume for grids apart from UnstructuredGrid.