Fix "Flow" does not have "_in_edges"

In old versions of oemof.network, Edge was subclass to Node.
Thus, the Flow had the attribute "_in_edges" though there was
no Edge connected to an Edge. Now, the Edge (sand thus Flow)
is cleaned up in oemof.network. As attributes starting with "_"
are ignored  anyway, this commit does not even change any behaviour.

src/oemof/solph/processing.py

@@ -528,7 +528,7 @@ def detect_scalars_and_sequences(com):
         attrs = [
             i
             for i in dir(com)
-            if not (callable(getattr(com, i)) or i.startswith(exclusions))
+            if not (i.startswith(exclusions) or callable(getattr(com, i)))
         ]
 
         for a in attrs: