NR module: prerequisites in LTE stack v2

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.

The difference from v1 is that we found a way to solve the problem of Tag at RLC and PDCP. Instead of updating my previous MR, I have created a new one, which already includes the fixes to all the comments made in the previous version.

Please consider it for inclusion.