Option to add "mandatory_passage" in searoute computation

[jutogashi]

Hi,

Thanks for the package!

Often, I'm interested in comparing the difference between two routes, for example:

• What is the distance from Houston to Haldia via cape?
• What is the distance from Houston to Haldia via Suez?

In the first case, it's easy to force that, as I can do all available passages - 'Suez'.

However, in the second case, I have to consider that, for a ship to go through suez, it also has to go through "gibraltar" and "babalmandab"

It would be nice to have a simple option only to consider paths that go through a passage, instead of thinking about what must be restricted every time.