Tidy DataFuture argument types (#3742)

The task ID is never optional: a DataFuture always comes from a Parsl
task. (in the same way that an AppFuture always has a task record)

Long ago, Files could be specified as strings. This behaviour was
removed slightly less long ago. Right now, typeguard should be detecting
if a string is passed, because of the type signature, and so the
explicit type check for strings should be unnecessary. This PR removes
that explicit type check.

The future being tracked by the data future is not necessarily an
AppFuture: it can be any kind of future that completes to indicate the
represented file is now available. That is already reflected in the type
annotations and code, but this PR fixes the docstring.

# Changed Behaviour

none

## Type of change

- Code maintenance/cleanup

parsl/app/futures.py

@@ -2,7 +2,6 @@
 """
 import logging
 from concurrent.futures import Future
-from typing import Optional
 
 import typeguard
 
@@ -39,24 +38,23 @@ def parent_callback(self, parent_fu):
             self.set_result(self.file_obj)
 
     @typeguard.typechecked
-    def __init__(self, fut: Future, file_obj: File, tid: Optional[int] = None) -> None:
+    def __init__(self, fut: Future, file_obj: File, tid: int) -> None:
         """Construct the DataFuture object.
 
         If the file_obj is a string convert to a File.
 
         Args:
-            - fut (AppFuture) : AppFuture that this DataFuture will track
-            - file_obj (string/File obj) : Something representing file(s)
+            - fut (Future) : Future that this DataFuture will track.
+                             Completion of ``fut`` indicates that the data is
+                             ready.
+            - file_obj (File) : File that this DataFuture represents the availability of
 
         Kwargs:
             - tid (task_id) : Task id that this DataFuture tracks
         """
         super().__init__()
         self._tid = tid
-        if isinstance(file_obj, File):
-            self.file_obj = file_obj
-        else:
-            raise ValueError("DataFuture must be initialized with a File, not {}".format(type(file_obj)))
+        self.file_obj = file_obj
         self.parent = fut
 
         self.parent.add_done_callback(self.parent_callback)