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. 
Iterating over all entities will be very expensive and there should be done some database side filtering to reduce the dataset, as it is to be expected that there will a great number of parts.