v2.1 feature: Geometrical operations

[BenjaminMidtvedt]

I propose operations between geometries/scatterers based on set theory. Examples would be:

sphere_a = dt.Sphere(position=(-2, 0), radius=3*u.px)
sphere_b = dt.Sphere(position=(2, 0), radius=3*u.px)
dumbbell = dt.Union(sphere_a & sphere_b, position=(5, 5))

sphere_a = dt.Sphere(position=(-2, 0), radius=3*u.px)
sphere_b = dt.Sphere(position=(2, 0), radius=3*u.px)
saucer = dt.Intersection(sphere_a & sphere_b, position=(5, 5))

sphere_a = dt.Sphere(position=(0, 0), radius=3*u.px)
sphere_b = dt.Sphere(position=(0, 0), radius=1.5*u.px)
shell = dt.Difference(sphere_a & sphere_b, position=(5, 5))

These are extensions of the Composite feature.

TBD: Should they be operations? E.G * for union, / for intersection, - for differencve?
TBD: How to propagate intensity values? Maybe a property?