- ENB_PHY_OUTPUT_SIGNAL to trace output signal
- ENB_MAC_SCHEDULING_REQUEST to trace scheduling requests

This program extracts a given subframe from a recorded trace.

see https://gitlab.eurecom.fr/oai/openairinterface5g/issues/227

When the UE connects to the eNodeB and receives its IP address from the
network, it calls system() to set it in the linux kernel world. This call
is not done in a realtime thread, but in the NAS, which uses its own thread,
independent of the realtime processing.

In some situations this totally disrupts realtime processing.

It is difficult to know precisely why that happens, but it seems that calling
fork(), as system() does, in a multi-threaded program is not a good idea. (So
say several people on the internet.) It is not clear why the softmodem is
impacted, but it seems that fork() is really what triggers the disruption.
Several tests lead to that conclusion.

To fix the problem, we create a child background process very early in main()
(before anything else basically). Then instead of calling system(), the main
process sends the string to the background process. The background process
gets the string, passes it to system() and reports the success/failure back
to the main process.

This solution involves a lot of system calls, but calling system() in the
first place is not cheap either. As long as no realtime thread uses this
mechanism, things should be fine. Time will tell.

Prior to this commit, the following command failed to build dlsim:
    ./build_oai --phy_simulators -c

Choice has been made to define it in a .h file as a static inline
function.

We now have 4 versions of T_HEADER:
 - bad quality C++ version with time
 - good quality C version with time
 - bad quality C++ version without time
 - good quality C version without time

Adapt calling sites too.

When data comes from the user, it is good to check that
what we read is correct and warn the user if it's not.

we want those traces in the logs people send to us and
the group HEAVY is disabled in the wiki instructions.

looks better

I chose to remove it from common/utils/itti/itti_types.h,
maybe it's not the correct solution.

see issue #165

Normally it is used to display throughput.

The computation it does is to sum up the wanted value for the last
second (1000 TTIs) and send this value to the views associated to
this logger, at each TTI.

This view is used to display throughput (to come in next commits).

- change paint function:
  when the XY plot is resized we scale. Previously
  the last range was put in the middle of the new range
  (say when we increase the size).
  We may get aspect ratio changes if the resize is not identical
  vertically and horizontally, but I think this behaviour is more
  'natural'.
- fix a bug:
  the last horizontal tick label was printed to far on the right,
  out of the bouding box of the XY plot. This is not totally fixed
  in the case the label is larger than the XY plot. Now the part
  out of the bounding box will be printed on the left. No big deal,
  make the plot big enough. (Before, even if big enough you had a
  problem.)
- add a new vertical tick display, to be used for throughput mostly.
  See in enb.c the difference between 'input signal' and throughput
  plots (those throughput plots will come in later commits).

It's like a label but with fixed dimensions and maximum
characters' capacity at creation time.

This function is used to print rates (say '1kb/s' or '16Mb/s').

[-1 29] was not very pleasant for MCS 28 or no DCI (value is -1 in this case).
[-2 30] looks better.