added type global_theory -- theory or local_theory;
added begin/exit_local_theory;
removed theory_context;
renamed body_context_node to presentation_context;
removed copy (checkpoint twice instead -- avoids unrelated theories);
Ref System Logics HOL ZF Inductive AxClass TutorialI IsarOverview IsarRef IsarImplementation Locales LaTeXsugar IsarAdvanced/Classes IsarAdvanced/Codegen