Allow strings in `DataFuture` initialization
Allow strings in `DataFuture` initialization

Now that `File` is subclassed from `str`, `isinstance(File, str)`
will always evaluate as True. Fixes #185.

Parsl/parsl,swift-lang/swift-e-lab,swift-lang/swift-e-lab,Parsl/parsl,Parsl/parsl,Parsl/parsl