cylinder, area, volume, new: circumference

implemented:

• 2D and 3D square
• 3D pyramid
• 3D cone
• 2D and 3D cylinder
• area, volume and circumference for curved objects where it made sense
• contains for cylinder3 and pos3

unsure whether:

• to allow for non-right (oblique, "tilted") cylinders?
• to keep storing both base and top disk/segment of cylinder?
• radius should be stored explicitly or just in base/top?
• circumference should be renamed to perimeter or even be removed again?
• allow more constructions like ball(sphere), e.g. disk(circle) and vice versa?