Optional registry

[tdamsma]

Implements changes suggested in #485.

I already put it up here for review, but did not properly document the changes yet.

[stevepiercy]

This is looking good! Just a couple more lines to cover. Lint and docs don't like the docstrings, so I'll suggest improvements to fix those jobs.

[stevepiercy]

These changes should get lint and docs to pass.

[stevepiercy]

@tdamsma the functional tests started failing after the merge commit 51b5418. Can you take a look to see what needs fixing?

[tdamsma]

yes, I'll have a look. I switched computer three times in the last week, so need to take a moment to get a proper environment up and running. I can't see why/how 51b5418 could have changed anything for the functional tests though

[stevepiercy]

@tdamsma I approve your work. Would you please review my docstring improvements and documentation additions in the last commit? I think I got it right, but it helps to have a second pair of eyes. Thank you!

[tdamsma]

Nice improvements, please go ahead and finish it. I put some minor comments, feel free to ignore them

[stevepiercy]

I think we're done. w00t! Thank you @tdamsma!