a script that makes error ellipse plots of ensemble atcfunix files

[SamuelTrahanNOAA]

An ensemble error ellipse script with no proper home yet. The script will probably end up somewhere else, but at least this PR gives us a forum for code review and discussion.

Given an ensemble hurricane forecast, this script plots error ellipses. The ellipses are plotted at one standard deviation, tilted to match the spread of the dataset. It uses the eigendecomposition of the covariance matrix of lat & lon differences from the mean location. (See references.) Ellipses are plotted every 24 hours by default, but this can be changed.

I expect that, once this is put inside some larger package, the atcfunix reader and data structures will be replaced with whatever the rest of that package uses.

References:

Idea came from here:

• https://journals.ametsoc.org/view/journals/mwre/139/2/2010mwr3456.1.xml

How to do an error ellipse plot:

• https://users.cs.utah.edu/~tch/CS6640F2020/resources/How%20to%20draw%20a%20covariance%20error%20ellipse.pdf

[SamuelTrahanNOAA]

@KathrynNewman @CPKalb @mkavulich @willmayfield - This is the draft PR I mentioned with the error ellipse plotting script.

[mkavulich]

@SamuelTrahanNOAA This looks good to me, at least as a temporary home, as we discussed last meeting. If you mark this PR ready for review I'm happy to approve it. If you'd like a more thorough review of the new plotting script I can also test it out.

[SamuelTrahanNOAA]

Someone other than me should test the script.