swc: Add 'pre and post' label to header

Author: clbarnes

navis/io/swc_io.py

@@ -576,7 +576,7 @@ def _write_swc(x: 'core.TreeNeuron',
         """)
         if export_connectors:
             header += dedent("""\
-            # 7 = presynapses, 8 = postsynapses
+            # 7 = presynapses, 8 = postsynapses, 9 = both pre- and post-synapses
             """)
     elif not header.endswith('\n'):
         header += '\n'