'Two dimensional Constant model function'

@staticmethod
def evaluate(x, y, amplitude):

    if (amplitude.size == 1):
        x = np.empty_like(x, subok=False)
        x.fill(amplitude.item())
    else:
        x = (amplitude * np.ones_like(x, subok=False))
    if isinstance(amplitude, Quantity):
        return Quantity(x, unit=amplitude.unit, copy=False)
    else:
        return x
