[Opm] 2020.04-rc3

[Arne.Morten.Kvarving at sintef.no]

2020.04 rc3 builds for ubuntu bionic and ubuntu focal are available in the testing ppa.

Changes in rc3:

- Added JSON description for some new keywords and updated the
MissingFeatures list

- Fixed bug in GCONSALE and Group reduction

- Formatting fixes in RSM files

- Fixes to build against Dune master

- Build system fixes to OpenMP linking for external projects