SONFMode

  SONFMODE creates a filter mask structure and/or 
  sets the mode for marker filtering

  Implemented though SONFMode.dll

  FILTERMASK=SONFMODE(MODE)
  FILTERMASK=SONFMODE(FILTERMASK, MODE)

  INPUTS: FILTERMASK, if present, is a filter mask structure that will be
                copied to the output
           MODE is a string, currently 'OR', 'AND' or 'NOCHANGE'
                (not case sensitive) -
                N.B. only the 1st letter is examined. If 'nochange' is 
                set without an input filter mask, or no mode is specified,
                'AND' mode is selected by default.
  OUTPUT: A FILTERMASK structure with flags set according to MODE. 
            If a filter mask is provided on input, mask values will be
            copied from it to the output mask filter.


  Author:Malcolm Lidierth
  Matlab SON library:
  Copyright © The Author & King's College London 2005-2006