make waveform generator class saved in GWT meta data

This MR allows the CBCResult plot to work with non-standard waveform generator classes, as long as the new class takes the same initialisation arguments.

bilby/core/utils.py

@@ -998,6 +998,8 @@ class BilbyJsonEncoder(json.JSONEncoder):
             return {'__dataframe__': True, 'content': obj.to_dict(orient='list')}
         if inspect.isfunction(obj):
             return {"__function__": True, "__module__": obj.__module__, "__name__": obj.__name__}
+        if inspect.isclass(obj):
+            return {"__class__": True, "__module__": obj.__module__, "__name__": obj.__name__}
         return json.JSONEncoder.default(self, obj)
 
 
@@ -1036,7 +1038,7 @@ def decode_bilby_json(dct):
         return complex(dct["real"], dct["imag"])
     if dct.get("__dataframe__", False):
         return pd.DataFrame(dct['content'])
-    if dct.get("__function__", False):
+    if dct.get("__function__", False) or dct.get("__class__", False):
         default = ".".join([dct["__module__"], dct["__name__"]])
         return getattr(import_module(dct["__module__"]), dct["__name__"], default)
     return dct