Audit code for arguments that should be keyword-only

[tbekolay]

Is your feature request related to a problem? Please describe.

There are several objects that I've only every seen called with keyword arguments, and without them they're very hard to parse. I'd be willing to be bet that no one reading this will know what these do without looking them up.

nengo.Ensemble(10, 1, 2)
nengo.Connection(pre, post, 0.1, solver=LstsqL2(True, 0.2))

Describe the solution you'd like

Since we dropped Python 2.7 support, we can now use keyword-only arguments, which would make the above raise a TypeError.

We should go through the things listed in the frontend API and make arguments keyword-only where appropriate.

Additional context

This will be a breaking change, so this might be a good thing to do for Nengo 4.0.