Bugs fixed (printing of atomic disjunction, symetry of class instanciation instruction, clearer error messages), semantic support extended

* solutions may now contain only semantic information * possibility to create semantic macros from the metagrammar with XMG * possibility to split the produced grammar in several XML files * well formedness principles added: TAG validity of solutions are checked following 2 rules every intern node that is labelled with another mark as nadj or none lead to an invalid tree every tree with 2 foot nodes or more is invalid * bug fixed in the printing of atomic disjunctions * bug fixed in the Engine (symetry of the unification equations when it concerns extended class instantiation, ie when dot operator is used) * clarification of GUMP's error message when a metagrammar file is not found * bug fixed in the error messages when an included file is not found (line and column number) * addition of a warning message when a non-colored node is used with colored metagrammar * extension of XMG's language to define extern features, declare classes as semantic classes, and highlight some classes (cf trace summary in the GUI)