v0.3.0

• Add new par_contours method to ContourBuilder to generate contours in parallel (only available with the parallel feature flag).

• Fix assignment of interior rings when a polygon with a hole is contained in the hole of another polygon (on the same band).

• Use more idiomatic code for area and contains functions.

• Don't store cval on Cell struct.

• Avoid pulling and compiling serde_json when the geojson feature is not enabled.

• Improve documentation and README.

• Add executable example.