T hacks: add new tracer ant0 and a tool to plot ofdm data from usrp or rfsim (output or input)

The tool ant0 dumps IQ data to a file (or stdout) as float complex or short.

It is needed for a student's project. (Well, we could use record/extract, but it's simpler.)

The tool ofdm-plot is used to plot output or input signal from gnb in the frequency domain. May be useful to debug, why not.

Reviewer: please don't comment too much on the code itself, it's a hack. (But I will of course accept critics.)

(putting label 5G-NR, for a small modification in rfsim, adding two T traces)