Added formula

Author: jmafoster1

causal_testing/specification/metamorphic_relation.py

@@ -179,6 +179,7 @@ def to_json_stub(self, skip=True) -> dict:
             "effect": "direct",
             "mutations": [self.treatment_var],
             "expected_effect": {self.output_var: "NoEffect"},
+            "formula": f"{self.output_var} ~ {'+'.join([self.treatment_var] + self.adjustment_vars)}",
             "skip": skip,
         }