Base sink: add buffer util functions (#65)

* Pull waterfall get_plot_data binding to base_sink

And make is so returned array has proper length

* Add function to poll number of items in buffer of base_sinks

Merge pull request #43 from Notou/porting-3.9

Brings compatibility with 3.9 (and hopefully 3.10)

Support for bokeh up to 2.2.3 (#40)

Merge support for bokeh 2.0 (#39)

* Create a new bokeh do for each new connection

* Update the widgets

* Update plots

* Display options

* Remove startup prints and warnings

* Remove callback_throttle

* Javascript fix for max hold

* Restore maxhold option to vec sink

* Minor improvement to test flowgraphs

* Update readme

Final merge of GSoC work into master