Try using non-default-value keyword argument syntax for parsl_resourc…

…e_specification

This makes parsl_resource_specificiation mandatory rather than defaulting to {}

It might be better to allow this to befault to None, so that users can still omit it
if they want to not specify anything...

parsl/benchmark/perf.py

@@ -22,7 +22,7 @@ def load_dfk_from_config(filename):
 
 
 @parsl.python_app
-def app(parsl_resource_specification={}):
+def app(*, parsl_resource_specification):
     return 7