Changes associated with implementation of ACCESS gap filling:
 - this version labelled V2.8.1 and released for beta-testing
 - replaced explicit values of -9999 used for missing data with
   parameter constants.missing_value, itselfg set to -9999 in
   constants.py;
 - added the "missing_value" attribue to all variables at L1 stage;
   - note that netCDF4 returns a masked array for a variable if the
     "missing_value" attribute is set for that variable
   - added a test in qcio.nc_read_series to see if the returned
     series is a masked array and convert to ndarray with masked
     elements set to -9999 if required
 - major refactor of gfACCESS_plotdetailed to separate code for
   statistics and plotting
 - miscellaneous changes associated with polishing for the
   beta-release