GeoJSON writer

[jacobwhall]

This repo now includes a writers/geo folder which contains the groundwork for conversion of skyway element data types to geo types.

This issue is meant to track progress on the GeoJSON writer, the first to use these geo type conversions.

• Convert nodes
• Convert ways
  • Create polygons from ways if appropriate
• Convert relations
  • Handle superrelations
  • Handle multilinestrings
    • Merge connected ways in multilinestrings
  • Handle multipolygons
    • Ring Assignment
    • Ring Grouping
      • RG-1 Build containment matrix
      • RG-3 Find rings not contained by any other ring
      • RG-4 Find every ring contained by RG-3 ring but not any other
      • RG-5 Create polygons from inner rings with consistent tagging
      • RG-6 Merge connected inner rings
    • Multipolygon creation
• Gracefully (and predictably!) handle missing data, e.g. nodes missing from ways
• Write some tests

[jacobwhall]

This is mostly taken care of. I pushed some experimental code that mostly works, but needs to handle finer details (like way orientation when constructing ways) better.