astbuilder: Introduction of new construction services

[Delja]

Add two new services in the astbuilder module:

• Add a way in ModelBuilder to create AST and module entity at same time (create_method, create_class, create_attribute).

• Add a new property in MEntity create_ast_representation to return a ast representation of self.

[Delja]

@privat The create_ast_representation methods can be moved in the ast builder.

Regarding the methods of the NitBuilder class, there is not really a correct class to put them. The objective of the ModelBuilder is to instantiate the model according to the AST and the ASTBuilder has instantiated the elements of the AST. In our case, we do both at the same time. Maybe the module/class name is incorrect.