paths of length zero not added to network

[VincenzoPerri]

Adding paths of length zero to a path object, e.g:

p = pp.Paths()
for i in range(10):
    p.add_path(tuple([i]))

When then generate a network path:

n = pp.Network.from_paths(p)

we get an empty network instead of one of isolated nodes:

n.nodes
defaultdict(dict, {})