Add RectifiedLinear neuron type

A lot of this PR is testing changes, in preparation for adding the `RectifiedLinear` type to the `nl` and `nl_nodirect` test fixtures. The neuron type itself is just a small modification of the notebook example.

Unfortunately, this fails a lot of unit tests. We could add this in with some `RectifiedLinear` specific tests, like we did with the adaptive LIF classes. In any case, let's discuss Monday morning.