Restructure decision matrix

[agritheory]

Instead of the confusing tuple-keyed dictionary, let's use another layer of nesting:

"Item" : {
    "Purchase Receipt": [
    ("add_or_increment", "Purchase Receipt Item", "item_code", target.item_code, target),
    ]

This will allow for clearer access of the correct function(s) and to make it hookable in the future.

[Alchez]

Fixed in 44e15df (#3)