Consider deprecating obsolete visualizer operators

[bruno-f-cruz]

A few of the classes of the Visualizer's package have now been moved to the new GUI.ZedGraph. Since the support for UI is so recent in Bonsai, we should consider deprecating the Visualizers package to prevent confusing people as to which to use. Alternatively, we could make the Visualizers package a "meta-package", similar to Bonsai.Neuro or Bonsai.Expressions, that installs GUI and GUI.ZedGraph.

[glopesdev]

@bruno-f-cruz Althought the GUI elements in this package have been superseded, the basic visualizers for all numerical types still live here, together with a few other core designer types.

For that reason we should probably deprecate only the GUI elements for 2.9, rather than the whole package.

A more full review of the whole visualisation stack will be done for 3.0.

[bruno-f-cruz]

That makes sense!