build a CAR geometry from a bounding box

[xzackli]

This PR introduces the ability to build a CAR geometry out of two points that define a bounding box, i.e.

using Pixell
import Pixell: degree, arcminute

box = [10  -10;           # RA
       -5    5] * degree  # DEC

shape, wcs = geometry(CarClenshawCurtis, box, 0.5 * arcminute)

It also updates the fullsky geometry to accept units. I've defined Pixell.radian, Pixell.degree, and Pixell.arcminute helpers that are aliases for things from Unitful.jl.