Merge branch 'feature/support-cpgrid' into 'master'

CMakeLists.txt

bin/installexternal.sh

dumux/discretization/cellcentered/subcontrolvolume.hh

dumux/io/cpgridcreator.hh

patches/README

patches/opm-common-2017.04.patch

patches/opm-common-2017.10.patch

patches/opm-common-2018.04.patch

patches/opm-core-2017.04.patch

patches/opm-grid-2017.04.patch

patches/opm-grid-2018.04.patch

patches/opm-parser-2017.04.patch

patches/opm-parser-2017.10.patch

test/porousmediumflow/2p/implicit/CMakeLists.txt

test/porousmediumflow/2p/implicit/cornerpoint/CMakeLists.txt

...mediumflow/2p/implicit/cornerpoint/grids/hardsmall.grdecl

test/porousmediumflow/2p/implicit/cornerpoint/problem.hh

...porousmediumflow/2p/implicit/cornerpoint/spatialparams.hh

...mediumflow/2p/implicit/cornerpoint/test_2p_cornerpoint.cc

...iumflow/2p/implicit/cornerpoint/test_2p_cornerpoint.input