function?[A]=OMP（DXL）;?
%=============================================
%?Sparse?coding?of?a?group?of?signals?based?on?a?given?
%?dictionary?and?specified?number?of?atoms?to?use.?
%?input?arguments:?
%???????D?-?the?dictionary?（its?columns?MUST?be?normalized）.
%???????X?-?the?signals?to?represent
%???????L?-?the?max.?number?of?coefficients?for?each?signal.
%?output?arguments:?
%???????A?-?sparse?coefficient?matrix.
%=============================================
[n