Implement a continuous descent indicator for Flight

[xoolive]

Flight.continuous_descent(
    airport: str | Airport,
    ratio_threshold: float = 0.05  # maybe?
) -> bool

• remove any go around attempt,
• set a radius for "ASMA" area and identify top of descent (new issue?)
• first successful attempt with len(level) / len(descent) < 0.05
• raise an Exception or return False if the aircraft doesn't seem to land at that airport

cc @junzis