feat: add default_target_function

Create a default target function as described in Eq. 3 from the QN paper
(https://doi.org/10.1186/1752-0509-1-4).

Author: ReubenJ

test/qn_test.jl

@@ -85,7 +85,7 @@ end
                 qn_size,
                 domains,
                 max_eq_depth;
-                schedule = Asynchronous,
+                schedule=Asynchronous,
             )
             async_qn_system = create_qn_system(async_qn)
             step!(async_qn_system, 100)
@@ -103,7 +103,7 @@ end
     domains = [1:N for _ = 1:qn_size]
 
     async_qn =
-        sample_qualitative_network(qn_size, domains, max_eq_depth; schedule = Asynchronous)
+        sample_qualitative_network(qn_size, domains, max_eq_depth; schedule=Asynchronous)
     async_qn_system = create_qn_system(async_qn)
 
     grid = Tuple(range(0, 1) for _ = 1:qn_size)