NR module: prerequisites in LTE stack

This merge request starts from the changes that the mmWave people have done until December 2016. From this point, CTTC began its own NR module. I removed the features that belong to other merge requests (such as RLC low latency), and I stripped down the very minimum to be able to compile CTTC NR module inside ns-3-dev.

I have adapted many things taking into account the already existing LTE module, trying not to break things. However, I failed miserably for what regards the RLC and PDCP Tags. Since in mmWave and NR module the packets can be split at MAC level, these tags are not working anymore. This patchset removes the tag placement, and therefore the Rx traces will show "0" as the delay between the layers.