Allow calling find_network with type (bayesflow-org#200)

* in addition to strings, type (i.e., uninstantiated classes) are
  accepted as well

* allows easy use of user-defined networks

bayesflow/utils/dispatch/find_network.py

@@ -17,3 +17,10 @@ def _(name: str, *args, **kwargs):
             raise ValueError(f"Unsupported network name: '{other}'.")
 
     return network
+
+
+@find_network.register
+def _(cls: type, *args, **kwargs):
+    # Instantiate class with the given arguments
+    network = cls(*args, **kwargs)
+    return network