PR 574: use variadic template instead of initializer_list to implement fixed-size vector ctor from coefficients.

doc/snippets/Matrix_variadic_ctor_cxx11.cpp

@@ -0,0 +1,3 @@
+Matrix<int, 1, 6> a(1, 2, 3, 4, 5, 6);
+Matrix<int, 3, 1> b {1, 2, 3};
+cout << a << "\n\n" << b << endl;