Add mean and mean's unit test.

src/Function2D.h

@@ -51,6 +51,15 @@ namespace Aurora {
      */
     Matrix sum(const Matrix& aMatrix,FunctionDirection direction = Column);
 
+    /**
+     * 求矩阵平均值，可按行、列、单元, 目前不支持三维，不支持复数
+     * @param aMatrix 矩阵
+     * @param direction 方向，Column, Row, All
+     * @param aIncludeNan 是否包含nan
+     * @return
+     */
+    Matrix mean(const Matrix& aMatrix,FunctionDirection direction = Column, bool aIncludeNan = true);
+
 };