BUG: saving reflectivity

Fixing simple bug to do with saving the reflectivity field during a wind
retrieval. I am also adding another check during quality control which
attempts to address velocity correction artifacts in the lower levels,
typically below 2000 m altitude. The 4DD algorithm produces a poor
solution (e.g. radial velocities 2 to 3 times the Nyquist velocity) in
regions predominantly consisting of noise, i.e. low SNR regions. So the
gridding picks up some of these pixels when they are not masked by
reflectivity, and therefore can negatively affect the wind retrievals.