support track_order parameter when creating groups

[magland]

          if `track_order` is true when creating an `h5py.Group`, then when iterating through the objects within the group, they are returned in order of creation, otherwise it is in ascending alphanumeric order. PyNWB doesn't use this. But I don't think it would hurt to pass this argument to `self._the_group.create_group(name)` below.

Originally posted by @rly in #28 (comment)