Fix &DMatrix - DMatrix.

Fix #203.

src/structs/dmatrix_macros.rs

@@ -516,7 +516,16 @@ macro_rules! dmat_impl(
 
             #[inline]
             fn sub(self, right: $dmatrix<N>) -> $dmatrix<N> {
-                right - self
+                assert!(self.nrows == right.nrows && self.ncols == right.ncols,
+                        "Unable to subtract matrices with different dimensions.");
+
+                let mut res = right;
+
+                for (mij, res) in self.mij.iter().zip(res.mij.iter_mut()) {
+                    *res = *mij - *res;
+                }
+
+                res
             }
         }
 

tests/mat.rs

@@ -439,7 +439,9 @@ fn test_dmat_addition() {
         ]
     );
 
-    assert!((mat1 + mat2) == res);
+    assert!((mat1.clone() + mat2.clone()) == res);
+    assert!((mat1.clone() + &mat2) == res);
+    assert!((&mat1 + mat2) == res);
 }
 
 #[test]
@@ -471,7 +473,10 @@ fn test_dmat_multiplication() {
         ]
     );
 
-    assert!((mat1 * mat2) == res);
+    assert!((mat1.clone()  * mat2.clone()) == res);
+    assert!((&mat1 * mat2.clone()) == res);
+    assert!((&mat1 * &mat2) == res);
+    assert!((mat1  * &mat2) == res);
 }
 
 // Tests multiplication of rectangular (non-square) matrices.
@@ -537,7 +542,9 @@ fn test_dmat_subtraction() {
         ]
     );
 
-    assert!((mat1 - mat2) == res);
+    assert!((mat1.clone() - mat2.clone()) == res);
+    assert!((&mat1 - mat2.clone()) == res);
+    assert!((mat1  - &mat2) == res);
 }
 
 #[test]