Add support for passing in parameter instances as streams

[philippjfr]

Makes it possible to pass in a Parameter instance as a stream, which will internally be converted to a Params stream. Also adds some validation to ensure the same parameter is not subscribed to multiple times.

• Adds tests

[jbednar]

Looks great, and should help keep user code simpler.

[jlstevens]

Looks good. I suppose we don't have any ideas on how to remap parameter names?

One thought I had (probably not a good idea) would be to allow something like:

hv.DynamicMap(..., streams={'foo':bar.param.baz})

This isn't great as you can't really mix parameter instances and streams nicely. Alternatively:

hv.DynamicMap(..., streams=[position_stream, ('foo', bar.param.baz)])

But I'm not hot on introducing more tuple formats.

[philippjfr]

Looks good. I suppose we don't have any ideas on how to remap parameter names?

Yeah, it's unfortunate, but I don't want to rush things and come up with something we will hate later so I think this a safe first cut.

[jlstevens]

Looks good! Much happier with the new warning.

[jlstevens]

Tests are green. Merging.