Adding voids to profile outside of constructor throws error

[secretlyagoblin]

Describe the bug
The pattern

var circle = Polygon.Circle(10);
var innerCircle = Polygon.Circle(9).Reversed();
var profile = new Profile(circle);
profile.Voids.Add(innerCircle);

throws an error as the voids list has not been instantiated.

Expected behavior is for this to just work as voids by default is a list of 0 elements.

Workaround of profile.Voids = new List<Polygon>(){innerCircle}; is fine for now but might trip up a newcomer and isn't the nicest.

[ikeough]

Closed in #295.