Merge branch 'feature/update-to-current-opm-release' into 'master'

[cornerpoint] adapt to current Opm release 2016.04

With Opm release 2016.04, the module dune-cornerpoint has been
renamed to opm-grid. Interfaces changed without deprecation. There
doesn't seem to be a preprocessor macro to check for the employed
Opm version. Therefore, drop support for older versions and only
support 2016.04.

See merge request !182