Beam simulation for rfsimulator

A simplified beam switching simulation for rfsimulator. This works
mainly in 1 gNB - N UEs scenarios. This is not a real beamformer with
antenna, channel and propagation model. The way it works is that the
peers inform each other about the beams they transmit in, and on
reception before channel modelling, the beam data is combined based on
preconfigured rx_beam/tx_beam gain matrix.

Usage:
 - Add --rfsimulator.enable_beams to enable beam simulation
 - Add --rfsimulator.beam_gains <comma separated Toepliz form matrix of gains in dB>.

   Example list: 0,-5,-10

   Resulting matrix:

   [[0, -5, -10]
    [-5, 0, -5]]
    [-10, -5, 0]]

 - Add --telnetsrv if you want to control the beams manually. The command in telnet
   is "rfsimu setbeam <beam_map>", where beam_map is a uint64_t controling the
   reception beam and transmission beam. If the application is switching the beams
   automatically the beams will be overwritten as soon as the application chooses to
   switch beams.
Co-authored-by: Laurent Thomas <laurent.thomas@open-cells.com>