correct typo on store function

pykkn/storage.py

@@ -43,7 +43,7 @@ class Storage:
     # def is_overwritable(self):
     #     return self.storage_path.exists()
 
-    def store_HDF5(self, root=h5py.File):
+    def store_HDF5(self, root=None):
         '''sub-method of store() for HDF5 output
 
         Parameters
@@ -103,7 +103,7 @@ class Storage:
                     for i in eval(attr):
                         i.store_HDF5(root=s_grp)
 
-    def store_json(self, root=dict):
+    def store_json(self, root=None):
         '''sub-method of store() for JSON output
 
         Parameters