If undo=TRUE this will undo a previous normalization. Otherwise, just normalize.
Value
Under default, a matrix with mean-zero and variance-one columns.
If shift and scale are specified, a matrix with columns shifted by -m and divided by s.
If undo=TRUE, and shift and scale are specified, an un-normalized matrix with column means m and standard deviations s.
In the special case where x is a simple_triplet_matrix and m=0, columns are scaled by s but left unshifted and the function returns a simple_triplet_matrix.