Add Density class to handle computation of density for Lazy objects #68

Currently we say the density is only a real number.
The idea is to represent the density as sections of circles.

Hence, the containers for Lazy evaluators #66 can compute the density value.

The initial motivation is:

• Suppose a SimpleShape A overlaps the SimpleShape B in a edge
• Pick up a point on that intersection
• Individually, computing the density would give 0.5 (smooth edge)
• But if LazyAnd is implemented, the density would be A.density(point) & B.density(point)