Mac flexran improvements

Multiple MAC-related and FlexRAN-related improvements:

• Proportional fair and maximum throughput scheduling algorithms for the default scheduler
• Dynamically modify the PLMN list from FlexRAN
• Dynamically connect to/disconnect from core networks
• (Re-) add static slicing -- other slicing algorithms will follow

@defosseu

1. The scheduling schemes (1st point) are configurable from the config file/command line. I think it would be useful to test the proportional fair scheduling algorithm in at least one one test. Should I just add a command line switch or do that from the config file, so that people might be aware of it?

$ sudo ./lte-softmodem -O ~/confs/l2-simulator.conf --MACRLCs.[0].default_sched_dl_algo "proportional_fair_wbcqi_dl"

configuration:

MACRLCs = ({
  ...
  tr_s_preference  = "nfapi";
  tr_n_preference  = "local_RRC";
  scheduler_mode   = "default";
  default_sched_dl_algo = "proportional_fair_wbcqi_dl";
});

2. In my tests UL was not stable. This merge request does not change the ULSCH scheduling directly, so can you please have a specific look at UL performance after the run to see whether everything is ok?

@navid.nikaein