nulled estimates at DC

Resolved conflicts. rfsim not working.

Integration develop nr 2019w42

See merge request oai/openairinterface5g!691

Merge branch 'integration_develop-nr_2019w42' of https://gitlab.eurecom.fr/oai/openairinterface5g into integration_develop-nr_2019w42

Signed-off-by: Raphael Defosseux <raphael.defosseux@eurecom.fr>

Conflicts:
	openair1/PHY/CODING/TESTBENCH/ldpctest.c

Conflicts:
	executables/nr-softmodem.c
	executables/nr-softmodem.h
	targets/ARCH/rfsimulator/simulator.c

Conflicts:
	executables/nr-gnb.c
	openair1/PHY/NR_REFSIG/nr_dmrs_rx.c
	openair1/SIMULATION/NR_PHY/ulsim.c

 - gNB now only sends samples to USRP when DL slot. (should do the same for UE)
 - increased sf_ahead to 6
 - fixed timestamp for usrp_write at UE

- added nr_is_ssb_slot function

- start_rb was set to 30, and nb_rb was set to 50 with NB_RB_DL = 106.
- When the DC subcarrier is included in the RB allocation,
  this causes errors in FFT/IFFT operations arounf the DC subcarrier
- This leads to errors in unscrambling and decoding