Extend extreme point API with gravity support + weight constraints

[skjolber]

Make it so that Point3D returns the StackPlacements which will come in contact with a StackValue (if it is placed at the point).

Define the relationship between StackPlacements using an intermediate object (i.e. graph style) in all directions. Use to calculate

• whether a box is within weight constraints for all underlying StackPlacements
• sideways support stability measurement
• allow for partial support
• stability measure (i.e. are underlying placement supporting enough of the box?)

Try to limit (exponential?) traversal of the graph .

• Cache max weight constraints for each StackPlacement, so that impossible values can be excluded quickly
• Possibly update constraints for each placed box

[skjolber]

Current extreme points have been refactored so that multiple implementations can co-exist.