(Non-multipolygon) relation support

[systemed]

This PR enables tilemaker to take account of relations in Lua scripts. There are two new (but linked) pieces of functionality:

• relation_scan_function reads relation tags and memberships into memory, so that they can be considered by member ways in way_function
• relation_function constructs discrete geometries from entire relations, including as multilinestrings

This means tilemaker can now support route and boundary relations, among others. Support for boundary relations is added to the OMT-compatible Lua script in this PR. The new functions are documented in a separate RELATIONS.md file.

Each relation type is sui generis so I anticipate this won't be the end of the story! As this evolves we may need to change the Lua interface for relations (semver purists look away now ;) ). But this should cope with the majority of cases.

The following aren't supported in this PR and could be considered at a later date:

• relation roles (moderately easy, may require changing the Lua interface)
• nested relations (quite hard)
• nodes in relations (moderately easy)

[leonardehrenfried]

Nice work!

Out of interest, where does FindInRelation search when the way is part of multiple relations?

[systemed]

It searches the current relation in the iterator controlled by NextRelation. In this example:

while true do
  local rel = way:NextRelation()
  if not rel then break end
  print ("Part of route "..way:FindInRelation("ref"))
end

then it'll show each ref in turn - NextRelation advances the current relation each time.

The idea is that you use it as part of a loop scanning through all the relations. If you need to store all of them, then you can create a Lua table and populate it as part of this loop.

[leonardehrenfried]

I've just tested this code with one of my data sets and it works great - well done!

I need to work on some styling first before I can share a screenshot but I'm really pleased already!

[systemed]

Ok, good to go I think!

[leonardehrenfried]

Because everybody loves a screenshot, here is what I built with this: national cycle routes in Norway.