Implement optional nodes on ways mode

[nyurik]

There is an optional feature LocationsOnWays available on OSM PBF files. It can be generated with osmium add-locations-to-ways ..., initially described here, and documented at osm wiki.

This PR implement support for this feature in the same way as way.refs().

• Unit tests have been updated to test for required and optional features
• Added refs() tests
• added approx equality (could be used for all other tests too)
• created a test file structure to keep track of what file has what features
• a few helper utilities

Should nodes() return None if the file has no LocationsOnWays set?

[b-r-u]

Wow, thanks for implementing that! I still need a little time to look into the code first, but I like that you even included a test file. I think it's okay to not return an Option if it just returns an empty iterator if the feature is not active.

[nyurik]

Sure thing, let me know how i can help with the review. There is a GeoRust discord channel you may want to join, or OSM-US Slack -- lots of fun OSM & Rust related discussions :)