Conductance of the empty set should return clear error message

[pmacg]

Requesting the conductance (or bipartiteness) of an empty set should return a meaningful error message to the client.

Minimal working example

>>> import sgtl.graph
>>> graph = sgtl.graph.cycle_graph(10)
>>> graph.conductance([])
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/home/peter/wc/py-sgtl/sgtl/graph.py", line 144, in conductance
    return 1 - (2 * self.weight(vertex_set_s, vertex_set_s, sets_are_equal=True)) / self.volume(vertex_set_s)
ZeroDivisionError: division by zero

For the bipartiteness case:

>>> graph.bipartiteness([], [])
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/home/peter/wc/py-sgtl/sgtl/graph.py", line 159, in bipartiteness
    return 1 - 2 * self.weight(vertex_set_l, vertex_set_r) / self.volume(vertex_set_l + vertex_set_r)
ZeroDivisionError: division by zero

Desired Behaviour

Something like:

>>> import sgtl.graph
>>> graph = sgtl.graph.cycle_graph(10)
>>> graph.conductance([])
<snip>
ValueError: conductance of the empty set is undefined