add NaN value set for Matrix; add return of URT of a matrix

matrix/Matrix.hpp

@@ -412,6 +412,11 @@ public:
         setAll(1);
     }
 
+    inline void setNaN()
+    {
+        setAll(NAN);
+    }
+
     void setIdentity()
     {
         setZero();
@@ -512,6 +517,13 @@ Matrix<Type, M, N> ones() {
     return m;
 }
 
+template<size_t M, size_t N>
+Matrix<float, M, N> nans() {
+    Matrix<float, M, N> m;
+    m.setNaN();
+    return m;
+}
+
 template<typename Type, size_t  M, size_t N>
 Matrix<Type, M, N> operator*(Type scalar, const Matrix<Type, M, N> &other)
 {