A parser to read .cg files.

A .cg file follows the following rules : 1. Each line defines either an object, a morphism or a composition law entry. 2. The following strings are reserved : ' -','-> ',' = ' 3. To define an object, write a line containing its name. 4. To define an arrow, the syntax "source_object -name_of_morphism-> target_object" is used, where "source_object", "target_object" and "name_of_morphism" should be replaced. 4.1. If an object mentionned does not exist, it is created. 4.2. The spaces are important. 5. To define a composition law entry, the syntax "source_object1 -name_of_first_morphism-> middle_object -name_of_second_morphism-> target_object1 = source_object2 -name_of_composite_morphism-> target_object2" is used, where "source_object1", "name_of_first_morphism", "middle_object", "name_of_second_morphism", "target_object1", "source_object2", "name_of_composite_morphism", "target_object2" should be replaced. 5.1 If an object mentionned does not exist, it is created. 5.2 If a morphism mentionned does not exist, it is created. 5.3 You can use the tag ID/ in order to map a morphism to an identity.