Getter function for the diag elements of slice (#112)

matrix/Slice.hpp

@@ -119,6 +119,16 @@ public:
         }
     }
 
+    Vector<Type, P<Q?P:Q> diag()
+    {
+        Slice<Type, P, Q, M, N>& self = *this;
+        Vector<Type,P<Q?P:Q> res;
+        for (size_t j = 0; j < (P<Q?P:Q); j++) {
+            res(j) = self(j,j);
+        }
+        return res;
+    }
+
     Type norm_squared()
     {
         Slice<Type, P, Q, M, N>& self = *this;