add empty util

Author: LarsKue

bayesflow/utils/empty.py

@@ -0,0 +1,13 @@
+class empty:
+    """
+    Placeholder value for arguments left empty
+
+    Usage:
+
+    def f(x=empty):
+        if x is empty:
+            # we know the user did not pass x
+        if x is None:
+            # the user could have passed None explicitly
+
+    """