Added event method to DynamicMap

[jlstevens]

When prototyping the streams system, one thing we were wondering about was how to easily access streams from a DynamicMap in order to call update. The problem is you either need to keep a handle on the stream object or would need to use some mechanism to access the stream via some (optional) assigned name or by index. This poses some difficult problems and this PR suggests an alternative approach that bypasses this issue completely.

The event method on a DynamicMap does not deal with stream objects, just stream parameters. The appropriate streams that require updating are computed behind the scenes which works well as the identity of the streams is often of less interest to the user than the set of stream parameters (these are the keyword arguments passed to the DynamicMap callback).

If nothing else, the event method is is useful for debugging it will always allow print statements to appear in the notebook. Note that this isn't true for direct Bokeh interaction streams which sends such print statements to the web console:

Earlier on, I proposed that the update method of streams could also be renamed to event for consistency - unfortunately update already exists on DynamicMap as it inherits the method from HoloMap otherwise I would have called this method update too.

I no longer feel this is particularly important. This PR will mean that users rarely use an update method in any context: it is rarely used by users on HoloMaps or DynamicMaps and this new event method means users won't have to use the update method on streams either. That said, I'm still open to renaming update on streams to event if we feel this is better for overall consistency.

Lastly, I will want to have a tutorial demonstrating the event method as a way of building visualizations with streams. In particular, I will want a notebook demonstrating it with a visualization using a mix of kdims and dimensioned streams.

[philippjfr]

Looks good and it's what we discussed. Ready to merge?

[jlstevens]

If you are happy leaving streams with an update as opposed to event method, then I think it is ready. As the streams API is new, we can still do whatever renaming we want.

[philippjfr]

Yes, I'm happy with that. Merging.

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