Add function to viz_tools to plot average wind direction arrow(s)

[douglatornell]

Original report by Doug Latornell (Bitbucket: douglatornell, GitHub: douglatornell).

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It would be nice to have a function in the viz_tools module that would overlay 1 or more average wind direction arrows on surface or depth slice plots. For multiple arrows the plot region would be subdivided into equal regions; e.g. quarters. To do so the function probably needs to accept rows and cols arguments to describe how it is to be subdivided for the arrows. The arrows should be calculable from the u and v wind stress fields in the *grid_U.nc and *grid_V.nc files.