Patch for realistic RACH model in LTE model

This patch introduces a realistic model for RACH procedure in the LTE model. It modifies several classes of the LTE stack, from PHY layer up to the application. Moreover some helper classes to perform measurements and an example are added. 
The main ideas and modeling choices are explained in this paper: http://arxiv.org/pdf/1601.05098v2.pdf
Only PfFfMacScheduler is supported. There is an option to choose between the ideal model (the current implementation) and the realistic one, so that if the simulation involves only UE devices in connected mode the burden of RACH procedure can be avoided.

In the PHY layer the main issue was handling the reception of more than one message in the same time/frequency slot by the eNB. This is addressed in lte-spectrum-phy and related classes.Moreover the UE and eNB phy classes are modified to allow the transmission of a real message with PRACH preamble, which is added as a possible lte-control-message. MAC and RRC layers are modified accordingly, and contention resolution + message3 are added. PfFfMacScheduler schedules specific time/frequency resources to the PRACH preamble transmission slot. Power control class is extended to perform power control for PRACH preamble transmission as required by the standard.