UNSPSC catalog as a GORM tree-like structure
Refactoring of Level, code & getCode() renamed

- Field code renamed to levelCode (to use code to store the full code)
- Method getCode() renamed to calculateCode()
Additional field to store the full Level code
Cascading for deletes in all catalog tree-nodes
Catalog table name set to unspsc_catalog

The table name catalog could be problematic in some DBMS