Enhancement 16 tm rrc reconfig

This branch contains enhancements regarding signalling of the transmission mode
It also contains fixes for TM2, which now works with commercial phones
TM3 also works, but only with single layer
TM4 needs further improvements in uplink signalling (will be done in another branch)
All tests that pass for develop also pass for this branch (see https://gitlab.eurecom.fr/oai-internal/openairinterface5g/builds/560)

See merge request !41