Addition of pill-like aperture?

[ysBach]

I have made and been using the pill type aperture. (here. I named it PillBox as it is a combination of elliptical and rectangular("box") aperture, but I think Pill is a better name)

Pill type aperture is useful for trailed objects (non-sidereal observation like asteroids and comets).

A figure from Fraser et al 2016 (http://dx.doi.org/10.3847/0004-6256/151/6/158) with some annotations: Their package is TRIPPy

Thanks to elliptical aperture in photutils, I could make the rounded parts at the ends in elliptical shape. This has been used in some publications I was involved in (e.g., Ishiguro, Bach et al. MNRAS (2022)). One difficulty is that, as I heavily followed the internal form of other apertures (such as EllipticalAperture class), when the internal API in photutils changes, some users and my class students complain my package raises ImportErrors. Thus, I think it maybe beneficial if this PillAperture and related codes can be included into photutils.

If you think it is reasonable, I will try to make a PR (may take a few weeks though). How do you think?