Clean up the state of the Pan and Zoom Tools

[corranwebster]

Problem Description

We currently have a plethora of different Pan and Zoom tools (PanTool, PanTool2, ZoomTool, BetterZoomTool, TrackingPanTool, DragZoom, etc.)

We should audit, find out which ones are working as expected, and make them the default (this might be a breaking change, depending on exactly what is needed).

See also #6.

Expected behavior:

Developers should have a clear choice for Pan and Zoom tools to use, possibly with one or two clear alternatives that have distinctly different (but common) behaviour. Tools should be based on Enable tools where practical.

[aaronayres35]

As an update:
BaseZoomTool and SimpleZoom were removed in #670

Also, the ZoomTool currently exposed in chaco.tools.api just links to the BetterSelectingZoom class also exposed in the chaco.tools.api.
BetterZoom (also in api) is the base class for BetterSelectingZoom. There is also TrackingZoom (which subclasses ZoomTool which is actually just BetterSelectingZoom) in the api. Lastly, there is still DragZoom which subclasses BetterZoom and DragTool from enable.

pan_tool2.PanTool is not exposed in the api, and also I'm not seeing it used internally in chaco anywhere currently.