Makes distinction between functions more clear.

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
Apply threshold pressure for all dp
The threshold pressure is applied for all dp.
The sign of the threshold pressure is given by the sign of the dp.
sim_fibo_ad*: change the default for the use_cpr parameter from false to true

SPE9 and Norne are basically unusable if UMFPack is used as linear solver...
Merge pull request #197 from andlaus/use_cpr

sim_fibo_ad*: change the default for the use_cpr parameter from false to...
Merge pull request #193 from blattms/fix-176

Makes distinction between functions more clear. (#176)
Merge pull request #196 from totto82/bugfix_thpress

Apply threshold pressure for all dp