'Two dimensional Disk model function'

@staticmethod
def evaluate(x, y, amplitude, x_0, y_0, R_0):

    rr = (((x - x_0) ** 2) + ((y - y_0) ** 2))
    result = np.select([(rr <= (R_0 ** 2))], [amplitude])
    if isinstance(amplitude, Quantity):
        return Quantity(result, unit=amplitude.unit, copy=False)
    else:
        return result
