Explicitly create the netflux_basin mask array.
specify the season in reduced_variable construction
- In several functions, allow for data without time axes, as from climatology files.
- Add season variable to class reduced_variable, and pass it on to filetable.find_files().
 - detect climatology files, if in NCAR history tape format
- in method find_files(), if the variable is restricted to a particular season (normally the
  case), look first for matching climatology files.  Return time-dependent files only if no suitable
  climatology files can be found.
 - For a climatology file, return the season in place of the time range, for use in find_files().
- Put data for all files in a single climatology file (formerly it was one file per
   variable.)