Empty probes are Python lists instead of ndarrays

[arvoelke]

This is very minor but it's a difference between nengo and nengo_dl that might crop up in rare/special circumstances (discovered when looping and resetting probes inside of the simulator context).

In Nengo an empty probe array is a NumPy ndarray. But in NengoDL it is a Python list. I'd expect the behaviour to be the same with the NengoDL simulator since it is a drop-in replacement.

A minimal example:

import nengo
import nengo_dl

with nengo.Network() as model:
    p = nengo.Probe(nengo.Node(0))

with nengo_dl.Simulator(model) as sim:
    # sim.step()
    print(sim.data[p].shape)

Raises: AttributeError: 'list' object has no attribute 'shape'.

Switching nengo_dl.Simulator to nengo.Simulator instead outputs the shape: (0,).

Likewise, uncommenting the sim.step() (with either simulator) outputs the shape: (1, 1).