Get the default data for a charm.

        @param charm_id The charm's id.
        @param channel Optional channel name.

def charm(self, charm_id, channel=None):
        '''Get the default data for a charm.

        @param charm_id The charm's id.
        @param channel Optional channel name.
        '''
        return self.entity(charm_id, get_files=True, channel=channel)