Merge branch 'feature/box-adaptive' into 'master'

dumux/implicit/adaptive/adaptionhelper.hh

dumux/implicit/adaptive/gridadapt.hh

dumux/implicit/adaptive/gridadaptproperties.hh

dumux/implicit/adaptive/gridadaptpropertydefaults.hh

dumux/porousmediumflow/2p/implicit/adaptionhelper.hh

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

test/porousmediumflow/2p/implicit/lensproblem.hh

test/porousmediumflow/2p/implicit/lensspatialparams.hh

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

test/porousmediumflow/2p/implicit/test_boxadaptive2p.input

test/references/lensbox-reference.vtu

test/references/lensboxadaptive-reference.vtu

test/references/lensccadaptivepointsource-reference.vtu