Add some operator() to cv_image for compatibility with mmod loss. (#900)

* Add some operator() to cv_image for compatibility with mmod.

* Update documentation

dlib/opencv/cv_image.h

@@ -80,6 +80,32 @@ namespace dlib
             return reinterpret_cast<const pixel_type*>( _data + _widthStep*row);
         }
 
+        inline const pixel_type& operator()(const long row, const long column) const
+        {
+          DLIB_ASSERT(0<= column && column < nc(),
+              "\tcont pixel_type& cv_image::operator()(const long rown const long column)"
+              << "\n\t you have asked for an out of bounds column "
+              << "\n\t column: " << column
+              << "\n\t nc(): " << nc()
+              << "\n\t this:  " << this
+              );
+
+          return (*this)[row][column];
+        }
+
+        inline pixel_type& operator()(const long row, const long column)
+        {
+          DLIB_ASSERT(0<= column && column < nc(),
+              "\tcont pixel_type& cv_image::operator()(const long rown const long column)"
+              << "\n\t you have asked for an out of bounds column "
+              << "\n\t column: " << column
+              << "\n\t nc(): " << nc()
+              << "\n\t this:  " << this
+              );
+
+          return (*this)[row][column];
+        }
+
         long nr() const { return _nr; }
         long nc() const { return _nc; }
         long width_step() const { return _widthStep; }

dlib/opencv/cv_image_abstract.h

@@ -157,6 +157,30 @@ namespace dlib
                   of this image
         !*/
 
+        inline const pixel_type& operator()(
+            const long row, const long column
+        ) const
+        /*!
+            requires
+                - 0 <= row < nr()
+                - 0 <= column < nc()
+            ensures
+                - returns a const reference to the pixel at coordinates (row, column)
+                  of this image
+        !*/
+
+        inline pixel_type& operator()(
+            const long row, const long column
+        )
+        /*!
+            requires
+                - 0 <= row < nr()
+                - 0 <= column < nc()
+            ensures
+                - returns a reference to the pixel at coordinates (row, column)
+                  of this image
+        !*/
+
         cv_image& operator= ( 
             const cv_image& item
         );