z-score rescaling

% z-score rescaling.
% Input:
% 'data' : input data matrix (nSample X nFeatures, if dim = 1) or (nFeatures X nSamples, if dim = 2)
% Optional inputs:
% 'flag' : 0 {default} using sample std (N-1)
% 1 using population std (N)
% 'mean_array', 'sd_array' : (1 X nFeatures) optional inputs of mean and sd
% for the calculation of z-score. If not provided, mean and sd will be
% calculated based on the input 'data'.
% 'dim' : {defalt = 1} dimention along which the averaging and calculation
% of sd is performed for z-scoring.
% Output:
% 'zcoredData' : (nSample X nFeatures) output data after z-score rescaling.
% 'mean_array', 'sd_array' : (1 X nFeatures, if dim = 1) or (nFeatures X 1, if dim = 2) arrays for mean and sd
% WR Chen 03/16