Add/register one filter.

        Args:
            filter_name (str): Filter name used inside :program:`Jinja2` tags.
            filter_ref: Reference to the filter itself, i.e. the corresponding :program:`Python` function.
            force (bool): If set to ``True``, forces the registration of a filter no matter if it already exists or not.

        Note:
            The list of user added/registered filters can be retrieve with :mth:`registered_filters_list`

def register_filter(self, filter_name, filter_ref, force=False):
        """
        Add/register one filter.

        Args:
            filter_name (str): Filter name used inside :program:`Jinja2` tags.
            filter_ref: Reference to the filter itself, i.e. the corresponding :program:`Python` function.
            force (bool): If set to ``True``, forces the registration of a filter no matter if it already exists or not.

        Note:
            The list of user added/registered filters can be retrieve with :mth:`registered_filters_list`
        """
        if not force and (filter_name in self.filters_list()):
            self.log_warning("Extension %s already exist, ignore redefinition." % ext_in)
            return

        self.__jinja2_environment.filters[filter_name] = filter_ref