chnage pplacer threads to 1 in processor.py

Fixes rhysnewell#231 (comment)

aviary/modules/processor.py

@@ -99,7 +99,7 @@ def __init__(self,
         self.output = os.path.abspath(args.output)
         self.threads = args.max_threads
         self.max_memory = args.max_memory
-        self.pplacer_threads = min(int(self.threads), 48)
+        self.pplacer_threads = min(int(self.threads), 1)
         self.workflows = args.workflow
         self.request_gpu = args.request_gpu