Add SKU and StorageItems based basic storage mangement

This PR rewrites the storing of on_stock items based on StorageItems, which are combinations created by a Part and a particular StoragePlace, e.g. a shelf. 

Based on that there has been added a function not yet in usage to list all Parts having StorageItems with the on_stock number as well as there minimum stock. 
I'll accept the PR, but will create a high priority issue for the next version, as this will be a real show stopper if you parts will grow to a larger number.