Moves common length, depth and width attributes and methods into Dimensionable module.
Adds Container class.
Creates PlacedBoxCollection class to further extract adding placed boxes.
Refactors Boxify

* Sets STARTING_LEVEL constant
* Delegates container dimension, wasted space and volume of placed box methods to newly added Container class instance
* Makes use of PlacedBoxCollection method to add placed_boxes
* At the end of packing, set placed_boxes for container instance
Moves method to Box.
Fixes delegation syntax.