PR 572: Add initializer list constructors to Matrix and Array (include unit tests and doc)

- {1,2,3,4,5,...} for fixed-size vectors only
- {{1,2,3},{4,5,6}} for the general cases
- {{1,2,3,4,5,....}} is allowed for both row and column-vector

failtest/initializer_list_2.cpp

@@ -0,0 +1,16 @@
+#include "../Eigen/Core"
+
+#ifdef EIGEN_SHOULD_FAIL_TO_BUILD
+#define ROWS Dynamic
+#define COLS Dynamic
+#else
+#define ROWS 3
+#define COLS 1
+#endif
+
+using namespace Eigen;
+
+int main()
+{
+  Matrix<int, ROWS, COLS> {1, 2, 3};
+}