General hook for ModelStorage modification

[ced (Cédric Krier)]

@ced

check_modification(cls, records, values=None) which is called only by ModelStorage.write and ModelStorage.delete at the beginning.

We have some checks that are only to be enforced for external user and other checks that should always be enforced for data integrity.
So I think we should follow the proposal of Verify access right only for RPC and have this API:

• check_modification(cls, records, values=None, external=False) which is called without check access but external is set to True when check access is True.