Make python examples executable and add shebang (#1181)

python/example/network_ring.py

+#!/usr/bin/env python3
+
 import arbor
 import pandas, seaborn
 from math import sqrt

python/example/single_cell_model.py

+#!/usr/bin/env python3
+
 import arbor
 import pandas, seaborn # You may have to pip install these.
 

python/example/single_cell_multi_branch.py

+#!/usr/bin/env python3
+
 import arbor
 import seaborn
 import pandas

python/example/single_cell_swc.py

+#!/usr/bin/env python3
+
 # NOTE: deprecating spherical roots changes the behavior of this model.
 # There is no soma, because only the root sample has tag 1, which will be
 # ignored as it is always the proximal end of any cable segment.