define a schema for relation members

[k-yle]

Closes #134

This is my proposal for a schema to define the roles of a relation. In summary:

• presets can have a new field called relation
• to avoid duplication, there is also relationCrossReference, similar to stringsCrossReference
• each role is defined with a default (en-US) label, just like presets & fields.

Example for type=boundary:

{
  "relation": {
    "allowDuplicateMembers": false,
    "members": [
      {
        "roles": { "inner": "Inner" },
        "geometry": ["line", "area"],
        // no matchTags
      },
      {
        "roles": { "outer": "Outer" },
        "geometry": ["line", "area"],
        // no matchTags
        "min": 1
      },
      {
        "roles": { "admin_centre": "Capital", "label": "Label" },
        "geometry": ["point", "vertex"],
        "matchTags": [{ "place": "*" }],
        "max": 1
      }
    ]
  }
}

Regarding matchTags:

• (no value) means any tags
• [{}] also means any tags (but this is weird, just exclude matchTags)
• [] means no tags (it will never match, this is unhelpful)
• [{ a: 1, b: 2 }] means a=1 AND b=2
• [{ a: 1 }, { b: 2 }] means a=1 OR b=2

[1ec5]

Thanks!

[k-yle]

updated the PR description about matchTags. Instead of the confusing [{}] syntax, you can just omit matchTags.