Changes to flowfield calculations / descriptions.

[jwookey]

This commit updates the calculations of a geographical decription of the flowfield to be (a) more intuitively defined, and (b) with an interface more consistent with the other parts of the code. Specifically:

• The geographical reference frame for the flow field is updated to east-north-up.
• The model field is updated from u_geog to u_enu to make it more descriptive.
• The interface is updated to (lon,lat) pairs rather than (lat,lon).

It also adds an example of plotting the flowfield on a map.

Fixes issues #107 and (partially) fixes #43

[eejwa]

Looks good to me in terms of the code changes and tests! Thanks again for doing that. I think there's an issue in the example where the path to an example terra model isn't on GitHub. I'm not sure how to get around this and where we got to with hosting an example model online somewhere for download. @andreww @anowacki what do you think?

[eejwa]

@jamespanton93 seems to get around this by making a NetCDF file from scratch from the function make_old_file() in the test_convert_files.py file. I think for the flow conversion we'd need to have a u_xyz field instead of Velocity_x and maybe a specific pattern rather than random?

[jwookey]

Thanks for looking over the code @eejwa. For the moment (and for the sake of clarity) I've removed the example from the pull request so it just covers the changes to the flow field. I'll look at how James does it, but I think we indeed talked about having an example model in some form with the distribution, so I'll hold off generating stuff until we decide.