Pruning of radar gates complete!

Now radar gates are removed (pruned) when (1) they are above the "top of
the atmosphere" and (2) they are too far to be captured during any
distance-dependent weighting, like for Cressman and Barnes schemes. In
the old mapping routine there was an additional pruning step which
removed radar gates where reflectivity was missing. However I am not
sure this is actually desired, since there are certain fields (e.g.,
cross correlation ratio) that the user likely wants mapped everywhere,
since they do a good job at indicating non-meteorological echoes, noise,
and (somewhat) cloud edges. More to come...