function?r?=?randnab（abMN）
%Generate?the?normal?distribution?with?mean?a?and?standard?deviation?b.
%?a:?input?parameter?mean
%?b:?input?parameter?standard?deviation
%?M:?input?parameter?row?number
%?N:?input?parameter?column?number
%?r:?output?parameter?the?random?matrix
%?Example:?
%r?=?randab（1