returns the rml mapping filepath

        rml_name(str): Name of the rml mapping to retrieve
        rtn_format: ['filepath', 'data']

def get_rml(self, rml_name, rtn_format="filepath"):
        """ returns the rml mapping filepath

        rml_name(str): Name of the rml mapping to retrieve
        rtn_format: ['filepath', 'data']
        """

        try:
            rml_path = self.rml_maps[rml_name]
        except KeyError:
            if rml_name in self.rml_maps.values() or os.path.exists(rml_name):
                rml_path = rml_name
            else:
                raise LookupError("rml_name '%s' is not registered" % rml_name)
        if rtn_format == "data":
            with open(rml_path, "rb") as fo:
                return fo.read()
        return rml_path