vis.revcorr.calc_significance

 CALC_SIGNIFICANCE - generate the p-value array and the rescaled matrix
 for plotting by taking in the sta and spike_num

  [P_VAL, RESCALE] = CALC_SIGNIFICANCE(STA,SPIKE_NUM)

  Inputs:
   STA - the STA array for calculation
   SPIKE_NUM, the number of spikes in the original json file

  OUTPUTS:
   P_VAL - an array with the same size as STA array, and record the
   significance of each pixel
   RESCALE - an array with the same size as STA array. It takes the value
   of P_VAL array and rescale it for plotting purposes.