Return type string, compatible with numpy.

def update(self, dt=-1):
        """
        Return type string, compatible with numpy.
        """
        self.library.update.argtypes = [c_double]
        self.library.update.restype = c_int
        if dt == -1:
            # use default timestep
            dt = self.get_time_step()
        result = wrap(self.library.update)(dt)
        return result