New recommended API

[michaeldeistler]

I accidentally overrode #169 so this reimplements it:

New API:

comp = nx.Compartment()
branch = nx.Branch(comp, nseg=4)
cell = nx.Cell(branch, parents=[-1, 0, 0])

Also possible is:

comp1 = nx.Compartment()
comp2 = nx.Compartment()
branch1 = nx.Branch([comp1, comp2])
branch2 = nx.Branch([comp1, comp2])
cell = nx.Cell([branch1, branch2], parents=[-1, 0, 0])

Therefore, the old API does not break. It is simply no longer the recommended way to do things.