Normalize encoders. Always.

Fixes #565.

Had to change `spa.test_thalamus.test_routing` as it failed with
this change. After looking at the plot, seemed that it was testing
values before settling; altered the points tested accordingly.
Break up test_reset and test_dt_dependence

We were doing too much with these tests, which will cause weird
failures for certain neuron models.

`test_reset` was broken up into two tests, one that tests that
neuron models reset properly and one that tests that learning rules
reset properly. Similarly, `test_dt_depenence` was also broken up
into two tests, one that uses LIFs and all learning rules and one
that doesn't use learning but tests all neuron types.
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.
Add RectifiedLinear neuron type

In doing so, noticed some errors in the rectified linear example
notebook. Fixed those errors in this commit.
Add Sigmoid neuron type

And enable it for all `nl` and `nl_nodirect` tests.
Add Izhikevich neuron type

Unlike the Sigmoid and RectifiedLinear types, the Izhikevich neuron
is not really suitable to be added to the `nl` and `nl_nodirect`
fixtures; while it is possible to decode functions from Izhikevich
ensembles, it requires a lot more tweaking and care. Instead, there
is an explanatory notebook, as well as a short test that ensures that
we can simulate (without crashing) several sets of commonly used
Izhikevich model parameters. Note that without the tweaks that are
commented on in `Izhikevich.step_math`, even this simple test might
blow up.
Fix slicing pre node with a function on connection

- Can now connect from a node with slicing and a function
- Added test for this case in test_connection
Fix Pep8 in test_connection
Can pass list of functions to ensarray.add_output
Added test for multiple functions on EA output
Rebase to master