GP bug when only using subclasses of return type

[AnnanFay]

If you set return type to a class and only have nodes which return subclasses then you will run into an issue when generating trees because ret_type is never added to the type dicts.

Potential fix, after line 263:

    self.primitives[ret_type]
    self.terminals[ret_type]

This initialises the entries to empty lists and the type will then appear when iterating over keys in _add.