Fix datarange2d bounds reset

The `_high_setting` and `_low_setting` attributes are unused and untested aside from the `refresh()` and `reset()` methods, which do not work. They are not properly coupled to their respective counterparts in the `DataRange1D` instances. This results in unexpected behavior when using tools that can knock them out of sync.

For example using the `ZoomTool` on a plot will manually set the axes' ranges, but calling `plot.range2d.reset()` will not set them back to `auto`.

This patch:
  1. Changes `DataRange2D.reset()` to use the public `self.*_setting` properties
  2. Removes the vestigial `self._*_setting` traits. No tests needed to be changed.
  3. Tests that setting bounds manually and then resetting puts them back to `'auto'`.