Adjusted diff_id_field to be decorator or function for user to call on existing classes.
Adjusted diff_id_field to be decorator or function for user to call on existing classes.

kstrauser/schematics,NerdWallet/schematics,ee08b397/schematics,eatfirst/schematics,not-napoleon/schematics,nKey/schematics,mlyundin/schematics,promptworks/schematics,not-napoleon/schematics,kaiix/schematics,promptworks/schematics,ee08b397/schematics,openprocurement/schematics,kstrauser/schematics