Plot arcs of ellipses instead of discrete points for ecliptic and milky way planes.

[dylanagreen]

Currently desipoint plots the MW and Ecliptic planes by plotting discrete points along those curves. This was a hacky method that was designed based on the method by which I plot these planes in the web viewer, since I couldn't figure out how to plot arcs in d3 at the time. This issue is applicable to both the web interface and the python package, but is more prevalent in the python package, as seen in this example image:

I propose instead to:

1. Fit an ellipse to the visible points (i.e. points above the horizon)
2. Plot only the visible arc of that ellipse using the matplotlib arc patch.

Point 1 is considerably more difficult than point 2.

[dylanagreen]

Per aforementioned commit, I have addressed this partially. These curves are not true ellipses and fits do not converge when using SVD. The best bet is to plot them as continuous lines, but I am weary that if any point is out of order that the line will cut straight back across to the other side of the image (as has happened before). This can probably be addressed by sorting the x,y points by their theta angle, with special care paid to the -pi to pi transition.