Hint the return type of `SourceTree.get_tree()`

[MatthewHambley]

It would be good to get a type hint on the return of SourceTree.get_tree() but each implementation will return a different type. For instance Fortran will return an fParser Program object but C++ would return something else.

This issue implies an investigation into the best way to handle this.

[MatthewHambley]

This issue no longer seems relevant. SourceTree.get_tree is unhinted but is also abstract. FortranSource.get_tree is hinted to return an fparser.Program and PlainText.get_tree returns a Generator.