add turf

https://github.com/Turfjs/turf/

Turf is a modular library for processing and analyzing geospatial data. It runs in node.js or in browser. Turf aims for maximum simplicity, and uses GeoJSON as its interchange format. It performs common geo tasks like:

- Finding the intersection of polygons
- Geometry smoothing and simplification
- Complex surface analysis
- Spatial statistical analysis and classification

Each submodule contains comprehensive docs, examples, and tests.