Checks to see if a list was passed in for indices

Takes care of the error encountered when doing something like: 

```
nengo.Connection(a[[0,2,3]], b[[0,3,4]])
```

Here's a test script:
```
import numpy as np

import nengo

N = 10
model = nengo.Model('test')
with model: 
    # input relays
    input0 = nengo.Node(output=[.5])

    a = nengo.Ensemble(neurons=N, dimensions=4)
    b = nengo.Ensemble(neurons=N, dimensions=5)
   
    # connect up 
    nengo.Connection(a[[0,2,3]], b[[0,3,4]])
    nengo.Connection(a[[0,2,3]], b[0:3])
    nengo.Connection(a[0:3], b[0:3])

    # probes
    in0_probe = nengo.Probe(input0, 'output')
    b = nengo.Probe(b, 'decoded_output', filter=.1)

sim = nengo.Simulator(model, dt=.001)
```