Need null default for roost polygons arg

[kaijagahm]

i.e. should not need roost polygons for networks that don't make use of them.

Should have a default null argument for e.g. creating co-flight networks.

Or, if co-flight networks do in fact require specification of roost polygons, then need to make that clear in the documentation for the wrapper functions.

[kaijagahm]

Feeding and flight do indeed require these, and I think it's fairly clear in the docs.