in fact it syncs the USRP time to GPS time when available and then derives the SFN from the USRP time.
a side effect is that now SFN does not necessarily start at 0 and this also required other changes
 - initialization of the future_ul_tti_request.
 - fixing a bug in get_samples_per_slot

This reverts commit 5679e4a3.

changes to support 768-point DFT/IDFT, renaming of 768-point DFT for DFT-precoding (4 DFTs per call)