node_slopes, aspect, hillshade

1. Created grid.aspect() in base to calculate node aspects either from
output of grid.node_slopes_using_patches() or direct from elevation
data.

2. Promoted Katy B’s grid.hillshade() from raster to base to calculate
node hillslopes. Some minor debugging. Its functionality is now more
flexible, but original code using it will still run as-is. It too can
take either output from grid.node_slopes_using_patches() and
grid.aspect(), or can work direct from elevation data.

3. All of slope, aspect, hillshade methods can now return either degree
or radian values, according to parameter ‘unit’.

4. grid.node_slopes() is now just an alias for
grid.node_slopes_using_patches().
Merge branch 'patches_and_slopes'
Merge pull request #69 from landlab/patches_and_slopes

node_slopes, aspect, hillshade
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).
Suppress git commit of gifs in examples folder

We don’t want any sample gifs produced by the video plotting driver to
sync on github.
Merge branch 'animation&vids'
Merge branch 'master' of https://github.com/landlab/landlab
refinement of video_out

add_frame now takes the usual plot **kwds to allow control of e.g.,
camp.

produce_video now allows override of min and max scale.
Merge branch 'animation&vids'