Changes associated with gap filling:
 - implemented automated processing over a user specified number
   of days.
 - put in a check at the start of L4 processing to make sure that
   all values of -9999 have a non-zero QC flag (flag=8), the
   check is done in qcio.nc_read_series().
 - xlDateTime now calculated when an Excel file is written using
   the current OS to determine datemode (=0 for Windows and Linux,
   =1 for Mac OSX).
Still heading for a beta release version at V2.8.1.