Improve the HDF module.

- Some test cases are added
 - Two tutorials are written to show how to use it.

modules/hdf/samples/create_groups.cpp

@@ -0,0 +1,60 @@
+// This file is part of OpenCV project.
+// It is subject to the license terms in the LICENSE file found in the top-level directory
+// of this distribution and at http://opencv.org/license.html.
+
+/**
+ * @file create_groups.cpp
+ * @author Fangjun Kuang <csukuangfj dot at gmail dot com>
+ * @date December 2017
+ *
+ * @brief It demonstrates how to create HDF5 groups and subgroups.
+ *
+ * Basic steps:
+ *  1. Use hdf::open to create a HDF5 file
+ *  2. Use HDF5::hlexists to check if a group exists or not
+ *  3. Use HDF5::grcreate to create a group by specifying its name
+ *  4. Use hdf::close to close a HDF5 file after modifying it
+ *
+ */
+
+//! [tutorial]
+#include <iostream>
+
+#include <opencv2/core.hpp>
+#include <opencv2/hdf.hpp>
+
+using namespace cv;
+
+int main()
+{
+    //! [create_group]
+
+    //! [tutorial_create_file]
+    Ptr<hdf::HDF5> h5io = hdf::open("mytest.h5");
+    //! [tutorial_create_file]
+
+    //! [tutorial_create_group]
+    // "/" means the root group, which is always present
+    if (!h5io->hlexists("/Group1"))
+       h5io->grcreate("/Group1");
+    else
+       std::cout << "/Group1 has already been created, skip it.\n";
+    //! [tutorial_create_group]
+
+    //! [tutorial_create_subgroup]
+    // Note that Group1 has been created above, otherwise exception will occur
+    if (!h5io->hlexists("/Group1/SubGroup1"))
+       h5io->grcreate("/Group1/SubGroup1");
+    else
+       std::cout << "/Group1/SubGroup1 has already been created, skip it.\n";
+    //! [tutorial_create_subgroup]
+
+    //! [tutorial_close_file]
+    h5io->close();
+    //! [tutorial_close_file]
+
+    //! [create_group]
+
+    return 0;
+}
+//! [tutorial]