Changing the names of base nodes and visitors

[peterwvj]

Overture uses ASTCreator to specify the VDM AST, but it also uses ASTCreator to specify the IR or the code generation AST. Some of the classes in the Overture code base both process nodes from the VDM AST and the IR, which requires either the VDM INode class or the IR INode class to be fully qualified in order to be able to distinguish between the two INode classes.

So it would be useful to be able to set the name of the top node via the AST specification file in order to avoid referring to the types using their fully qualified names. A similar issue occurs for the names of the visitor base classes.