MOD: radar quality control

Updating the quaility control (private) function in the winds module to
(hopefully) handle the masking correctly. We have to be careful that
fill values do not end up contaminating the quality control routines, at
the same time masking appropriate areas, e.g. non-meteorological echoes.
Furthermore, this routine should work for np.ndarray or
np.ma.core.MaskedArray arrays...