Remove use of ChacoPlotItem in attractors example

[aaronayres35]

closes #409
part of #404

This PR removes the use of ChacoPlotItem which has recently been removed on chaco master and will be unavailable with the next release of chaco.

[aaronayres35]

In regards to #404, the example currently "works" but it is lacking safeguards so a user can easily crash things...
For example, launch the app and and immediately set B=2. The Henon Map plot will disappear and the traceback in terminal is:

/Users/aayres/Desktop/envisage/envisage/examples/demo/plugins/tasks/attractors/model/henon.py:68: RuntimeWarning: overflow encountered in double_scalars
  point = array([y + 1 - self.a * x ** 2, self.b * x])

(At least the full application doesn't crash)

Alternatively, select Lorenz Map from the drop down, then set Time stop = 1000. The application will crash and you will see:

$ python run_attractor.py 
2021-05-19 08:48:09.827 Python[47176:955260] ApplePersistenceIgnoreState: Existing state will not be touched. New state will be written to (null)
Traceback (most recent call last):
  File "/Users/aayres/.edm/envs/envisage-test-3.6-pyqt5/lib/python3.6/site-packages/enable/qt4/base_window.py", line 243, in paintEvent
    self.handler.paintEvent(event)
  File "/Users/aayres/.edm/envs/envisage-test-3.6-pyqt5/lib/python3.6/site-packages/enable/qt4/base_window.py", line 78, in paintEvent
    self._enable_window._paint(event)
  File "/Users/aayres/.edm/envs/envisage-test-3.6-pyqt5/lib/python3.6/site-packages/enable/abstract_window.py", line 542, in _paint
    self.component.draw(gc, view_bounds=(0, 0, size[0], size[1]))
  File "/Users/aayres/.edm/envs/envisage-test-3.6-pyqt5/lib/python3.6/site-packages/enable/component.py", line 418, in draw
    self._draw(gc, view_bounds, mode)
  File "/Users/aayres/.edm/envs/envisage-test-3.6-pyqt5/lib/python3.6/site-packages/enable/component.py", line 799, in _draw
    self._dispatch_draw(layer, gc, view_bounds, mode)
  File "/Users/aayres/.edm/envs/envisage-test-3.6-pyqt5/lib/python3.6/site-packages/enable/container.py", line 280, in _dispatch_draw
    component._dispatch_draw(layer, gc, new_bounds, mode)
  File "/Users/aayres/.edm/envs/envisage-test-3.6-pyqt5/lib/python3.6/site-packages/enable/component.py", line 817, in _dispatch_draw
    handler(gc, view_bounds, mode)
  File "/Users/aayres/Desktop/chaco/chaco/base_xy_plot.py", line 477, in _draw_plot
    self._draw_component(gc, view_bounds, mode)
  File "/Users/aayres/Desktop/chaco/chaco/base_xy_plot.py", line 484, in _draw_component
    self._render(gc, pts)
  File "/Users/aayres/Desktop/chaco/chaco/lineplot.py", line 371, in _render
    render(gc, points, self.orientation)
  File "/Users/aayres/Desktop/chaco/chaco/lineplot.py", line 382, in _render_normal
    gc.stroke_path()
  File "/Users/aayres/.edm/envs/envisage-test-3.6-pyqt5/lib/python3.6/site-packages/kiva/agg/agg.py", line 1360, in stroke_path
    return _agg.GraphicsContextArray_stroke_path(self, *args)
OverflowError: Exceeded cell block limit
Abort trap: 6

Ideally we would restrict these values to non-breaking ranges. Also the latter traceback I believe is a known issue.
Seems related to enthought/chaco#474

[aaronayres35]

The old active_model's name is the old title that was used as the name for the previous plot.

I do think this naming is a little confusing, as title is not actually used as a title... In fact, prior to this PR it was not actually used at all! I found it convenient for naming plots so they could be easily deleted when we switch models.

Note that if we do not delete the old plot when we change model, things appear to work perfectly fine initially, however if you change models a few times eventually you see the

OverflowError: Exceeded cell block limit
Abort trap: 6

crash as mentioned in another comment. Deleting old plots prevents this

[rahulporuri]

LGTM