Add default `rates` and `gain_bias` function

In the `NeuronType` class. These defaults should work for rate-based
neurons (`rates` is based on what `LIFRate.rates` was doing already).

The `gain_bias` function is adapted from some of @tcstewar's code.
Test all neuron types' tuning/response curves

These plots are quite useful when implementing new neuron types,
and these tests don't take long.
Add RectifiedLinear neuron type