Loops should have finite width

[wtbarnes]

Currently, the loops in our AR have zero cross-section and thus effectively have the width of a pixel. There should be an option for allowing the loops to have variable cross-sectional area, i.e. expanding flux tubes.

One option for doing this would be to have multiple SkyCoord objects represent a single loop. As some function of s (e.g. 1/B), the loop would expand in the direction perpendicular to s. Thus, a given loop would have multiple SkyCoords that fanned out in s_perp1 and s_perp2.

Then, when it comes time to bin the quantities over a detector array, the weights are repeated along each strand in the flux tube and then all multiplied by 1/N, with N the number of strands in the flux tube.