Add Sigmoid neuron type

Includes the testing commits from #569, and includes (part of) the generic `gain_bias` calculation from [`nengo_brainstorm`](https://github.com/ctn-waterloo/nef-chip-hardware/blob/master/nengo_brainstorm/neuron_tuning.py#L60).

Still a WIP to look at Monday morning; in this case, the issue is that the sigmoid is between 0 and 1, but really we should be scaling it to be from 0 to max_rates; not entirely sure the best way to do that, since `step_math` doesn't have access to gains and biases (they're incorporated into `J`).