Implement self-assigment for transformation rules (MWW request)

Especially for objects

Steps to reproduce:

(1) look at mapping id: 59a3cefc06bffc346f6f5e42 in mww dme instance: http://mww.de.dariah.eu/dme/mapping/editor/59a3cefc06bffc346f6f5e42/
(2) Mapped concept from IDENT to Source/Type:

Source = {
. = @IDENT;
Type = “id”;
}

(from redmine: issue id 844, created on 2018-01-22tgradl)