Declare entry point for nengo reference simulator.

[jgosmann]

Description:
This adds an entry point for the reference simulator so that other Python packages (like nengo_gui) can discover the backend in a general way. Such an entry point should become best practice for all backends.

Motivation and context:
See description.

How has this been tested?

1. Checkout the branch and do a python setup.py develop.
2. Run the following Python code:

from pkg_resources import iter_entry_points
print([ep.load() for ep in iter_entry_points(group='nengo.backends')])

It will print "[<class 'nengo.simulator.Simulator'>]".

Where should a reviewer start?
Click "Files changed".

How long should this take to review?

• Quick (less than 40 lines changed or changes are straightforward)

Types of changes:

• New feature (non-breaking change which adds functionality)

Checklist:

• I have read the CONTRIBUTING.rst document.
• I have updated the documentation accordingly.
• I have included a changelog entry.
• I have added tests to cover my changes.
• All new and existing tests passed.

Still to do:

• Add some documentation, but we need some general backend developer documentation first. I'll create an issue for that.

[mention-bot]

@jgosmann, thanks for your PR! By analyzing the annotation information on this pull request, we identified @tbekolay to be a potential reviewer

[tbekolay]

It will print "[]".

I get [<class 'nengo.simulator.Simulator'>] which I'm pretty sure is actually what we want ;)

Aside from that, this looks good to me! I pushed a commit adding a test.

[jgosmann]

That's the markdown messing with me ... click the edit icon and you will see I put in in there. I'm innocent. I promise. 😇

[jgosmann]

It would also be possible to load a specific entry point without iterating over all. Might that be more appropriate for the test?

[tbekolay]

The advantage of loading them all (which shouldn't take long) is that we don't have to write a different test for each backend; also, failing this test will let backends know that they should also declare this entry point.

[jgosmann]

Good point!

[Seanny123]

Given that the only test that is not passing was fixed by #1134, this branch looks ready to merge.