Opens the record file.

def open(self):
        """Opens the record file."""
        if self.flag == "w":
            check_call(_LIB.MXRecordIOWriterCreate(self.uri, ctypes.byref(self.handle)))
            self.writable = True
        elif self.flag == "r":
            check_call(_LIB.MXRecordIOReaderCreate(self.uri, ctypes.byref(self.handle)))
            self.writable = False
        else:
            raise ValueError("Invalid flag %s"%self.flag)
        self.pid = current_process().pid
        self.is_open = True