Simplified how Event subscribers are handled

[jlstevens]

This PR offers clear and add_subscriber methods to Event and does away with _hidden_subscribers. The subscribers attribute is now in fact a read-only property which means the correct way to add subscribers is with the add_subscribers method.

Outstanding items to address:

• Clarify whether DynamicMap.event and Stream.update methods support names before or after renaming.
• Add more unit tests
• Add docstrings

[jlstevens]

@philippjfr I think it is ready for review - I'm expecting the tests to pass.

[philippjfr]

Addresses #1168

[philippjfr]

Maybe I'm not following the logic correctly in the utilities but won't this try to send the supplied kwargs to all the streams even though they may not apply? That's what the overlap bit was for I assume.

[jlstevens]

Thanks, that has helped me realize what we need:

• An initial loop over set(kwargs.keys()) - set(all_streams) to raise an exception for anything in that difference set: these are kwargs that cannot be applicable to any stream. Previously invalid kwargs were silently ignored and I think this is bad behavior unless you have reasons to say otherwise.

• util.rename_stream_kwargs(stream, {k:v for k,v in kwargs.items() if k in set(stream.contents.keys())}, reverse=True) to filter the kwargs to the applicable ones.

Do you agree that this is the right approach?

[philippjfr]

Yes, that seems right.

[jlstevens]

Fixed in 79b7de1

[philippjfr]

Looks good now, will merge when tests pass.

[jlstevens]

Tests now passing except for one transient error.

[philippjfr]

Will merge, since the PR build has passed.

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