`init_states` broken

The following returns a `KeyError: '[3] not in index'`
```python
cell = jx.Cell(jx.Branch(ncomp=1), parents=[-1, 0, 0])

net = jx.Network([cell]*2)
net.insert(Leak())

pre_loc = net.cell(0).branch(0).loc(0.5)
post_loc = net.cell(1).branch(0).loc(0.5)

connect(pre_loc, post_loc, IonotropicSynapse())

net.init_states() # BREAKS!
```

