FT_APPENDSPIKE combines continuous data (i.e. LFP) with point-process data
 (i.e. spikes) into a single large dataset. For each spike channel an
 additional continuos channel is inserted in the data that contains
 zeros most of the time, and an occasional one at the samples at which a
 spike occurred. The continuous and spike data are linked together using
 the timestamps.

 Use as
   [spike] = ft_appendspike(cfg, spike1, spike2, spike3, ...)
 where the input structures come from FT_READ_SPIKE, or as
   [data]  = ft_appendspike(cfg, data, spike1, spike2, ...)
 where the first data structure is the result of FT_PREPROCESSING
 and the subsequent ones come from FT_READ_SPIKE.