Force Entity.custom_properties to be a keyword argument

We do not want custom_properties to be set exidentially,
so the parameter should be named.

src/oemof/network/network/entity.py

@@ -58,7 +58,7 @@ class Entity:
         "custom_properties",
     ]
 
-    def __init__(self, label=None, custom_properties=None, **kwargs):
+    def __init__(self, label=None, *, custom_properties=None, **kwargs):
         self._label = label
         if custom_properties is None:
             custom_properties = {}