[cornerpoint] add capability to use cornerpoint grids

Introduce a grid creator that reads from a Petrel output / Eclipse input
file and generates a CpGrid that is offered by the OPM module
dune-cornerpoint. Enhance the fully-implicit cell-centered models to be
able to deal with cornerpoint grids, test the functionality. This patch
consists of several parts:

- patches/ and dune.module: the CMake buildsystems from Dune and OPM are
  not compatible. The patches change some CMake files in the required
  OPM modules opm-parser, opm-core and dune-cornerpoint in such a way
  that those modules can serve as a dependency for Dumux. In addition,
  some specific CMake options have to be passed to dunecontrol. See
  patches/README for details.

- dumux/io/: defines the CpGridCreator which behaves like any other grid
  creator that reads from a file. In addition, it offers the Eclipse
  input deck in form of a static function deck(). This deck can be used
  to extract further simulation parameters, most prominently porosity
  and permeability values.

- dumux/implicit/cornerpoint/: changes some of the cell-centered
  classes to be able to deal with a CpGrid. The changes in the
  ElementVolumeVariables and the FVElementGeometry are minor and
  required by the facts that a CpGrid doesn't offer codim-1/2 entities
  and that the geometries don't offer local-to-global mappings. The
  changes in the DarcyFluxVariables are made for enhancing the
  robustness of the two-point flux approximation by forcing the
  inter-element transmissibilities to be positive.

- test/implicit/2p: runs a fully-implicit two-phase immiscible model on
  a CpGrid. An element index with an injection rate can be given in the
  input file. The grid provided in grids/ consists of only four
  elements, but exhibits typical cornerpoint features like a
  nonconforming situation and general hexahedrons. A more realistic grid
  will be provided in dumux-lecture soon.

Reviewed by gruenich.



git-svn-id: svn://svn.iws.uni-stuttgart.de/DUMUX/dumux/trunk@15183 2fb0f335-1f38-0410-981e-8018bf24f1b0