FT_FREQINTERPOLATE interpolates frequencies by looking at neighbouring
 values or simply replaces a piece in the spectrum by NaN.

 Use as
   freq = ft_freqinterpolate(cfg, freq)
 where freq is the output of FT_FREQANALYSIS or FT_FREQDESCRIPTIVES and the
 configuration may contain
   cfg.method   = 'nan', 'linear' (default = 'nan')
   cfg.foilim   = Nx2 matrix with begin and end of each interval to be
                  interpolated (default = [49 51; 99 101; 149 151])

 To facilitate data-handling and distributed computing you can use
   cfg.inputfile   =  ...
   cfg.outputfile  =  ...
 If you specify one of these (or both) the input data will be read from a *.mat
 file on disk and/or the output data will be written to a *.mat file. These mat
 files should contain only a single variable, corresponding with the
 input/output structure.