Adds a method for assigning collision ranks to features.

Method

The idea of a collision rank is that it's a unique integer assigned to everything that gets labelled. See #988 for more background. This adds a new resource, spreadsheets/collision_rank.yaml, which is a YAML-encoded list of filters. The syntax of the filters is the same as the kind & min_zoom YAML in the yaml/ directory (hopefully this minimises the amount of duplication, and means we can share bug fixes and/or new features).

Each feature is checked against each filter in turn, and is assigned the collision_rank corresponding to the matching index. Indices start at 1, are incremented by one for each filter, and can have "reserved blocks" which skip the index counter onwards. For example, if collision_rank.yaml contains:

- kind: foo
- kind: bar
- _reserved: {from: 10, to: 20}
- kind: baz

Then a feature with:

• kind: foo would get collision_rank: 1,
• kind: bar would get collision_rank: 2,
• kind: baz would get collision_rank: 21.

Note that indices 3-9 are unused in this example, but if we added an entry after bar, then it would be assigned 3. The code asserts that indices are unique, so we can't accidentally overflow the reserved blocks. Both the from and to values are inclusive, hence baz is assigned the next index, 21.

Why not use a CSV

We've been managing sort_rank with CSVs, but the issue is that we often don't care about the exact value of the sort_rank, but want to easily insert values and re-order the following lines. Having the sort_rank explicitly in the file means that any such re-arrangement has a very large diff, and is hard to check visually. This is an experiment to see if we can make something that still gives the features we want, but without the drawbacks of the CSV.

Additionally, the CSVs tended to get mangled by Excel when loading and/or saving, as we were using a bunch of special characters to indicate things such as "any" * or "must be null" - .

Finally, CSV files can't contain comments, but YAML ones can. This will make future maintenance easier, as we can comment # this reserved block for X, Y or Z.

Connects to #988.