add functionality to declarative plotting

[kgoebber]

An initial attempt to address some of the functionality desired in issue #1016 by adding contour labels, colorbar, colorfill plots, and linetypes to contours. Added a test and a few elements to other tests to hopefully get coverage of the new elements.

• labels (Boolean)
• colorbar (Unicode string of orientation)
• linetype (Unicode string of matplotlib linestyles)
• ContourFillPlot() creates a class to do filled contours, based on the ContourPlot() class

For both labels and colobar I would love to add more options to how they plot, but I don't currently understand the traitlets well enough to figure it out. This is a start anyway.

Only issue I have had is when I plot contours with an ImagePlot, the spacing of the contour labels gets HUGE! But I was able to fix the issue when I reversed the order in the panel call.

This works

panel = MapPanel()
panel.plots = [contour, img]

But this does not work as desired

panel = MapPanel()
panel.plots = [img, contour]

Unable to fully diagnose why the ordering changes the impact, but it does.

Looking for any feedback on this PR.

[dopplershift]

In general this is going in the right direction. I think there are a few things that need cleaning up, and I'm concerned that the changed test isn't failing.

Huge 👍 on the addition, thanks for putting this together.

[kgoebber]

Don't know why the test_declarative_contour_options is failing - passes on my machine.