Merge branch 'integration-develop-nr-2020w03' into 'develop-nr'

Integration develop-nr 2020 week 03

See merge request oai/openairinterface5g!755

This integration branch includes the following MRs:

- MR 745 : Add support for different PUSCH DMRS modes
- MR 748 : Nr dlsch multi threading
- MR 752 : Merging develop tag 2019.w51 to develop-nr
- MR 753 : RedHat Entreprise 7.7 build support

IMPORTANT: This should be the last integration branch to develop-nr as the result of this MR will become develop