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
%??????????????????X?-?the?signals?to?represent
%??????????????????L?-?the?maximal?number?of?coefficient?for?representation
%??????????????????of?each?signal.
%?output?arguments:?A?-?sparse?coefficient?matrix.
%=========