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. 
Known issue. No better solution by now found.