Fix bug in H5File

Already opened groups were not closed when calling 'open_group'
again.

dune/dorie/solver/util_h5file.hh

@@ -30,7 +30,9 @@ public:
      *  \param verbose Verbosity of the output
      */
     explicit H5File(const std::string& file_path, const int verbose):
-        _verbose(verbose)
+        _verbose(verbose),
+        _file_id(-1),
+        _group_id(-1)
     {
         // set up property list for collective I/O
         hid_t h5_plist_id = H5Pcreate(H5P_FILE_ACCESS);
@@ -72,7 +74,13 @@ public:
      */
     void open_group(const std::string& group_path="./")
     {
-        // open the group
+        // close the group if we already opened it
+        if (_group_id >= 0) {
+            herr_t status = H5Gclose(_group_id);
+            assert(status > -1);
+        }
+
+        // open the new group
         _group_id = H5Gopen(_file_id, group_path.c_str(), H5P_DEFAULT);
         if (_group_id < 0) {
             throw std::runtime_error("Cannot open H5 group " + group_path);