Small enhancements to Scavo and ContestoScavo
Add ForeignKey on ContestoScavo for MaterialeInCassa.

And align field names to standard ones, providing a few
default values. In some cases there is not a 1:1 match
between field names and the exchange format so we use the
standard name as a prefix (e.g. two separate fields need
to be combined into one).
MaterialeInCassa: default value for NME, more verbose repr
Scavo: improved model, remove references from Cassa to Scavo.

DSCD is left for now. The data model is much clearer now.
The release notes of South 0.8 suggest to change requirements