Resolves a param and validates the ownership.

        :param param: param name or the param instance, which must
                      belong to this Params instance
        :return: resolved param instance

def _resolveParam(self, param):
        """
        Resolves a param and validates the ownership.

        :param param: param name or the param instance, which must
                      belong to this Params instance
        :return: resolved param instance
        """
        if isinstance(param, Param):
            self._shouldOwn(param)
            return param
        elif isinstance(param, basestring):
            return self.getParam(param)
        else:
            raise ValueError("Cannot resolve %r as a param." % param)