Implement slice as copy loop

matrix/Matrix.hpp

@@ -355,7 +355,13 @@ public:
     template<size_t P, size_t Q>
     Matrix<Type, P, Q> slice(size_t x0, size_t y0) const
     {
-        Matrix<Type, P, Q> res(&(_data[x0][y0]));
+        Matrix<Type, M, N> &self = *this;
+        Matrix<Type, P, Q> res; //default constructed
+        for (size_t i = 0; i < P; i++) {
+            for (size_t j = 0; j < Q; j++) {
+                res(i, j) = self(i + x0, j + y0);
+            }
+        }
         return res;
     }