Add information on formal models

src/test/resources/de/hhu/stups/codegenerator/StringLaws.mch

+/*
+From ProB Examples, originally used to test ProB,
+rewritten for B2Program
+*/
 MACHINE StringLaws
 DEFINITIONS
   laws ==  ((s1=s2 <=> s2=s1) &

src/test/resources/de/hhu/stups/codegenerator/SubstitutionLaws.mch

+/*
+From ProB Examples, originally used to test ProB,
+rewritten for B2Program
+*/
 MACHINE SubstitutionLaws
 SETS
  ID={aa,bb}

src/test/resources/de/hhu/stups/codegenerator/Train_1_beebook_TLC.mch

+/*
+Original model:
+J.-R. Abrial. Modeling in Event-B: System and Software Engineering. Cambridge University Press, 2010.
+
+Rewritten for TLC
+*/
 MACHINE Train_1_beebook_TLC
 SETS /* enumerated */
   BLOCKS={A,B,C,D,E,F,G,H,I,J,K,L,M,N};

src/test/resources/de/hhu/stups/codegenerator/Train_1_beebook_deterministic.mch

+/*
+Original model:
+J.-R. Abrial. Modeling in Event-B: System and Software Engineering. Cambridge University Press, 2010.
+*/
 MACHINE Train_1_beebook_deterministic
 SETS /* enumerated */
   BLOCKS={A,B,C,D,E,F,G,H,I,J,K,L,M,N};

src/test/resources/de/hhu/stups/codegenerator/Train_1_beebook_deterministic_MC.mch

+/*
+Original model:
+J.-R. Abrial. Modeling in Event-B: System and Software Engineering. Cambridge University Press, 2010.
+
+Model applies Partial Order Reduction manually
+*/
 MACHINE Train_1_beebook_deterministic_MC
 SETS /* enumerated */
   BLOCKS={A,B,C,D,E,F,G,H,I,J,K,L,M,N};

src/test/resources/de/hhu/stups/codegenerator/Train_1_beebook_deterministic_MC_POR.mch

+/*
+Original model:
+J.-R. Abrial. Modeling in Event-B: System and Software Engineering. Cambridge University Press, 2010.
+
+Model applies Partial Order Reduction manually
+*/
 MACHINE Train_1_beebook_deterministic_MC_POR
 SETS /* enumerated */
   BLOCKS={A,B,C,D,E,F,G,H,I,J,K,L,M,N};

src/test/resources/de/hhu/stups/codegenerator/Train_1_beebook_deterministic_MC_POR_v2.mch

+/*
+Original model:
+J.-R. Abrial. Modeling in Event-B: System and Software Engineering. Cambridge University Press, 2010.
+
+Model applies Partial Order Reduction manually.
+Rewritten version for B2Program
+*/
 MACHINE Train_1_beebook_deterministic_MC_POR_v2
 SETS /* enumerated */
   BLOCKS={A,B,C,D,E,F,G,H,I,J,K,L,M,N};

src/test/resources/de/hhu/stups/codegenerator/Train_1_beebook_deterministic_MC_POR_v3.mch

-MACHINE Train_1_beebook_deterministic_MC_POR_v3 // this version is very close to Train_1_beebook_tlc_POR. For instance, top-level ANY substitutions are not re-written to PRE as done in Train_1_beebook_deterministic_MC_POR_v2. DEFINITIONS are still inlined.
+/*
+Original model:
+J.-R. Abrial. Modeling in Event-B: System and Software Engineering. Cambridge University Press, 2010.
+
+Model applies Partial Order Reduction manually.
+Rewritten version for B2Program.
+This version is very close to Train_1_beebook_tlc_POR. For instance, top-level ANY substitutions are not re-written to PRE as done in Train_1_beebook_deterministic_MC_POR_v2. DEFINITIONS are still inlined.
+*/
+MACHINE Train_1_beebook_deterministic_MC_POR_v3
 SETS /* enumerated */
   BLOCKS={A,B,C,D,E,F,G,H,I,J,K,L,M,N};
   ROUTES={R1,R2,R3,R4,R5,R6,R7,R8,R9,R10}

src/test/resources/de/hhu/stups/codegenerator/sort_m2_data1000.mch

+/*
+Original Model:
+V. Rivera, N. Catano, T. Wahls, and C. Rueda. Code generation for Event-B. STTT, 19(1):31–52, 2017.
+*/
 MACHINE sort_m2_data1000
 // a version where some assertions and invariants are simplified/removed
 // so that TLC4B can infer a function type for f and g

src/test/resources/de/hhu/stups/codegenerator/sort_m2_data1000_MC.mch

+/*
+Original Model:
+V. Rivera, N. Catano, T. Wahls, and C. Rueda. Code generation for Event-B. STTT, 19(1):31–52, 2017.
+*/
 MACHINE sort_m2_data1000_MC
 // a version where some assertions and invariants are simplified/removed
 // so that TLC4B can infer a function type for f and g

src/test/resources/de/hhu/stups/codegenerator/tictac.mch

+/*
+From ProB Examples
+*/
 MACHINE tictac
 
 ABSTRACT_VARIABLES  square, turn