ensure_max_edge_length should process all trees

[jstriebel]

Current ensure_max_edge_length processes only trees that are part of a group. It should work on all trees instead.
The relevant lines seem to be https://github.com/scalableminds/wknml/blob/master/wknml/nml_utils.py#L101-L102

[jstriebel]

@valentin-pinkau Do you know if this would be an easy fix?

[daniel-wer]

It should also work with empty trees, see this stacktrace:

File "/usr/local/lib/python3.6/dist-packages/wknml/nml_utils.py", line 98, in ensure_max_edge_length
    max_id = detect_max_node_id_from_all_graphs(nml_graph)
  File "/usr/local/lib/python3.6/dist-packages/wknml/nml_utils.py", line 49, in detect_max_node_id_from_all_graphs
    max_id_of_current_tree = np.array(list(tree.nodes)).max()
  File "/usr/local/lib/python3.6/dist-packages/numpy/core/_methods.py", line 30, in _amax
    return umr_maximum(a, axis, None, out, keepdims, initial, where)
ValueError: zero-size array to reduction operation maximum which has no identity