Add module doc string for chaco.api

[aaronayres35]

This PR adds a doc string to the api module so that the objects show up in the autogenerated api docs for the api module.

Note, I followed the same grouping as already existed in the api (to keep the diff simple), but we way want to do some shuffling / reorg. For example, BandedMapper is not currently in the "Mappers" section. I can make these changes here if preferred, or leave them to a follow up PR.

Additionally, I am not sure what to do about the "Colormaps and Color Palettes" section (hence it being empty for now), as there are ~80 color maps. Suggestions are very welcome. In that sense this PR is not entirely complete, but I am flagging it as ready for review because I am no longer working on it. We will want to address this before it is merged though.

[aaronayres35]

ColormappedSelectionOverlay seems more like a visual component

[aaronayres35]

BandedMapper should be moved to mappers

[rahulporuri]

LGTM. I think this is a good start. We can merge this as-is for now and revisit this later.

[rahulporuri]

I was actually thinking of making the * imports from chaco.default_colormaps and chaco.default_colors explicit. It will crowd the api module (and the module docstring) but I think it's better than users having to dig through the source code to find what the available color maps are.

And we can move it to the end of the module (and the module docstring) to maintain readability.

[aaronayres35]

That makes sense. I will merge this PR and do that in a follow up PR shortly

[aaronayres35]

CI is failing on windows/pyqt.

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ERROR: test_dont_crash_on_click (chaco.tests.test_plot.TestEmptyPlot)
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Traceback (most recent call last):
  File "D:\a\chaco\chaco\.edm\envs\chaco-test-3.6-pyqt\lib\site-packages\chaco\tests\test_plot.py", line 134, in test_dont_crash_on_click
    [(1, 1), (25, 25), (50, 50), (100, 100)],
  File "D:\a\chaco\chaco\.edm\envs\chaco-test-3.6-pyqt\lib\contextlib.py", line 88, in __exit__
    next(self.gen)
  File "D:\a\chaco\chaco\.edm\envs\chaco-test-3.6-pyqt\lib\site-packages\traitsui\testing\tester\ui_tester.py", line 106, in create_ui
    process_cascade_events()
  File "D:\a\chaco\chaco\.edm\envs\chaco-test-3.6-pyqt\lib\contextlib.py", line 88, in __exit__
    next(self.gen)
  File "D:\a\chaco\chaco\.edm\envs\chaco-test-3.6-pyqt\lib\site-packages\traitsui\testing\_exception_handling.py", line 74, in reraise_exceptions
    raise RuntimeError(msg)
RuntimeError: Uncaught exceptions found.
=== Exception (type: <class 'AttributeError'>, value: '_QtWindow' object has no attribute 'handler') ===
Traceback (most recent call last):
  File "D:\a\chaco\chaco\.edm\envs\chaco-test-3.6-pyqt\lib\site-packages\enable\qt4\base_window.py", line 258, in leaveEvent
    self.handler.leaveEvent(event)
AttributeError: '_QtWindow' object has no attribute 'handler'

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This is the test that was just added in #636. I am not sure why this failure wasn't seen on that PR?