Returns a new DesignMatrix in which the columns have been
median-subtracted and sigma-divided.
For each column in the matrix, this method will subtract the median of
the column and divide by the column’s standard deviation, i.e. it
will compute the column’s so-called “standard scores” or “z-values”.
This operation is useful because it will make the matrix easier to
visualize and makes fitted coefficients easier to interpret.
* Standardizing a spline design matrix will break the splines.
* Columns with constant values (i.e. zero standard deviation) will be
A new design matrix with median-subtracted & sigma-divided columns.