drf_plot needs a feature to allow it to tolerate data gaps

[AB4EJ-1]

If we have a rf data collection system which occasionally has to be halted for some reason, there will be a data gap. Today, drf_plot halts when it finds this, which makes it hard to produce a full-day plot on a research-type system that has to be taken down sometimes. Request an option to tell drf_plot to simply fill in with zeros if data is missing or NaN.