Get celldata

  [S, extra_args]=get_celldata(cellid)
  [S, extra_args]=get_celldata(ratname,experimenter, date)
  [S, extra_args]=get_celldata(ratname,experimenter, daterange)
 
  A frontend to get timestamps and waveforms from the spktimes table that does some nice input
  parsing.   Useful to use in other functions to avoid having to parse
  inputs.
 
  S by default contains fields: cellid, sessiondate, and protocol_data
 
  needs more documentation!  
 
  cellid can be a single cellid or a vector of cellids
  date should be of the form "YYYY-MM-DD" or a relative date like -5
  daterange should be a numeric vector in relative form like -10:-1 or a
  cell array of date string of the from "YYYY-MM-DD"