Cleanup/component co2

Closes #489

See merge request !1034

[components][simpleco2] remove simpleco2 as it is not used anywhere and can be replaced by C02.hh when the correct table is used

Freeflow/kepsilon improvements

See merge request !1029

For wall functions, near-wall cells are always matchinPoints and should get a fixed value.

Freeflow/komegancni

See merge request !1028

Freeflow/komega

See merge request !1027

[math] Implement weighted means

See merge request !1024

[io] Use mapper size for determining length of output vectors

See merge request !1025

[poromechanics] pull fluid density out of volume variables

See merge request !1022

For the coupled, problems this removes the dependency of the volume
variables on the coupling context, which causes problems e.g. when using
the standard vtk output module. We now obtain the density where needed
in the local residual.

Fix/various compiler warnings

See merge request !1023

Feature/support cpgrid

See merge request !1011

`opm-material` is not needed anymore. Since Dumux checks itself for
`opm-common` in its `CMakeLists.txt`, changes to `opm-grid`'s
buildsystem giles aren't necessary anymore.