Add placeholder for gv output

src/wireviz/Harness.py

@@ -270,6 +270,11 @@ class Harness:
 
         return dot
 
+    @property
+    def gv(self):
+        # self.create_graph().save()
+        raise NotImplementedError
+
     @property
     def png(self):
         from io import BytesIO

src/wireviz/wireviz.py

@@ -189,6 +189,8 @@ def parse(yaml_input: str, return_types: (None, str, Tuple[str]) = None) -> Any:
                 returns.append(harness.tsv)
             elif rt == 'tsv_excel':
                 returns.append(harness.tsv_excel)
+            elif rt == 'gv':
+                returns.append(harness.gv)
             elif rt == 'harness':
                 returns.append(harness)
             else: