Add functions for piste routes

[zerebubuth]

This adds processing for giving a kind to piste routes, and a post-process filter for removing routes tagged as abandoned.

Connects to tilezen/vector-datasource#342.

@rmarianski could you review, please?

[rmarianski]

👍

I wonder if it makes sense to add support to the individual transforms/filters themselves to signal that a feature should be dropped, eg returning None for the shape. I almost added this for something in the past, but it got implemented in a different way so I never added support for it.

[zerebubuth]

Yeah, I think it makes sense to do that - we've got more than one filter now which is about dropping certain features. It's pretty easy to do with the post-processor, but I always feel like that's a pretty "heavyweight" solution to a "lightweight" problem.

[rmarianski]

I made a new issue to capture this: #99