Handling of geometry collections

[springmeyer]

Geometry collections are not supported directly in the vector tile spec: The spec only describes point, linestring, polygon, and unknown: https://github.com/mapbox/vector-tile-spec/blob/master/1.0.1/vector_tile.proto#L9-L12.

So, the correct way to encode geometry collections is to "explode" them into multiple features (yes, the feature attributes should be duplicated). This will be fixed as part of the merge of #94

For example: collection(multipoint,line) -> feature 1(multipoint), feature 2(line). But collection(mulitpoint,point) could likely be encoded as feature (multipoint)

[springmeyer]

Full support for exploding geometry collections now in place after #106. So, we properly support geometry collections being passed to the encoder, as well as geometry collections inside other geometry collections. They will simply get exploding into features with a single geometry type.