Change [] return type to reference

src/Matrix.cpp

@@ -470,7 +470,7 @@ namespace Aurora {
         return operatorMxA(&vdPowI, times, std::forward<Matrix&&>(matrix));
     }
 
-    double Matrix::operator[](size_t index) const { return getData()[index];}
+    double& Matrix::operator[](size_t index) { return getData()[index];}
 
     void Matrix::printf() {
         if(isNull())

src/Matrix.h

@@ -170,7 +170,7 @@ namespace Aurora {
         Matrix operator==(const Matrix &matrix) const;
 
         // sub
-        double operator[](size_t index) const;
+        double& operator[](size_t index);
 
 
         /**