Add a filter for a facet.

def add_filter(self, name, filter_values):
        """
        Add a filter for a facet.
        """
        # normalize the value into a list
        if not isinstance(filter_values, (tuple, list)):
            if filter_values is None:
                return
            filter_values = [filter_values, ]

        # remember the filter values for use in FacetedResponse
        self.filter_values[name] = filter_values

        # get the filter from the facet
        f = self.facets[name].add_filter(filter_values)
        if f is None:
            return

        self._filters[name] = f