Simplify libraw param mapping

Tweak error message

rawkit/options.py

@@ -38,7 +38,7 @@ def _prepare_func(self, func):
     def __call__(self, func=None):
         self._prepare_func(func)
         if func is None:
-            raise TypeError("option must be a method.")
+            raise TypeError("option should not be called except as a property")
         self.func = func
         return self
 
@@ -377,17 +377,8 @@ def _map_to_libraw_params(self, params):
         :param params: the output params struct to set
         :type params: :class:`rawkit.libraw.libraw_output_params_t`
         """
-        def get_dict_attr(obj, attr):
-            for obj in [obj] + obj.__class__.mro():
-                try:
-                    if attr in obj.__dict__:
-                        return obj.__dict__[attr]
-                except:
-                    pass
-            raise AttributeError
-
         for slot in self.__slots__:
             prop = slot[1:]
-            opt = get_dict_attr(self, prop)
+            opt = getattr(Options, prop)
             if type(opt) is option and getattr(self, prop) is not None:
                 opt.write_param(self, params)