vad plotting

[abkenyon]

Created a function to plot a Velocity Azimuth Display from NEXRAD Level 3 VAD files

[dopplershift]

This feature in general is going to force us to consider how MetPy consists of a set of re-usable pieces.

Just some notes for where I see this going:

1. We need to update for matplotlib 2.1 (ENH: Support x,y units for barbs/quiver matplotlib/matplotlib#8971), which will allow barbs() to accept times directly for x/y values.
2. The direct processing of the VAD product should probably become part of Level3File...somehow
3. Is the answer pandas? Xarray and its built-in multi-file support?

After that, we can see what remains in the plotting.

@abkenyon to be clear I'm not expecting you to make all the changes, but I wanted to capture the things I've thought of while trying to see how this fits exactly. If you are interested in making changes, though, I'm happy to help outline what those exactly might look like.

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