Two special-case treatments to allow CERES data: (1) reduce2scalar_seasonal_zonal
recognizes and changes its weird time units, (2) reduced_variable.reduce()
calls a new function special_case_fixed_variable to convert a CERES variable with
a nonstandard _FillValue attribute into a proper masked variable.

Moreover, reduce2scalar_seasonal_zonal ignores a time axis which consists of only
one missing value.  We have encountered this kind of (non-CF-compliant, malformed,
and just plain silly) data too many times.
Delete two debugging printouts.