EXTRAPOL N1 N2 N3 -- Extrapolate spectra in time
 Parameters: 
     
  N1    -- Slot number of first spectrum to average
  N2    -- Slot number of last spectrum to average
  N3    -- Slot in which to put results
  Danny Steeghs @ soton
  BETA VERSION
  
 Selection is done. 
 The spectra must have equal numbers of pixels, but no other properties
 in common. A warning is given if the wavelength scales differ but
 averaging still goes ahead
This command belongs to the class: time_series