report 0% overhead when none of the function results have returned

Previously it reported 100% which is not a good estimate either.

Author: basnijholt

adaptive/runner.py

@@ -180,6 +180,10 @@ def overhead(self):
         but is a rough rule of thumb.
         """
         t_function = self._elapsed_function_time
+        if t_function == 0:
+            # When no function is done executing, the overhead cannot
+            # reliably be determined, so 0 is the best we can do.
+            return 0
         t_total = self.elapsed_time()
         return (1 - t_function / t_total) * 100
 
@@ -426,7 +430,6 @@ class AsyncRunner(BaseRunner):
         overhead of the executor. Essentially, this is
         ``100 * (1 - total_elapsed_function_time / self.elapsed_time())``.
 
-
     Notes
     -----
     This runner can be used when an async function (defined with