Delete duplicate method in raster

We had two copies of raster.number_of_core_nodes. We don’t any more.
Return plot objects in imshow

1. imshow methods now all return plot objects, just like
matplotlib.imshow would. (Necessary to support video plotting)
2. You can now also use the “allow_colorbar=True” keyword to suppress
the addition of a colorbar to any of these plot methods. (Again,
necessary for video plotting)
create plot.video_out.py; test script

This commit creates the plot.video_out component. This allows automated
creation of video (gif) output from landlab runs.
At the moment, due to codec weirdness in the matplotlib library, only
gifs are supported. Individual users could tweak their own PATHs to
allow, e.g., mp4s.
landlab/examples/plotter_test.py is added as a test case for video
plotting.

At the moment, this is demonstrated to work for simple cases, but has
not been well tested for Voronoi grids (colorbar may fail) or
complicated cases where other, static plots are also being produced
(should be fine, but not tested).