How to handle non-plate entities?

[ormestad]

It might be needed to also catalogue non-plate entities such as boxes, tubes etc.

The slot variable for racks defines how many plates that fit into each position, so non-plate entities could be defined by how many plate units they occupy. This way it will be possible to accurately show multi slot positions as occupied if larger storage units are used. Also it may be possible to restrict larger units to only specific racks with multi-slot positions.