union geometries

[mapsam]

In order to more effectively solve deduping and cross-tile querying, it'd be great to union intersecting geometries (such as those split by a tile boundary).

This will require some work with boost and spatial-algorithms in order to move forward.

cc @artemp @flippmoke

[springmeyer]

👍 I think this will be a fun quest. I recommend looking into #55 next, because a potentially optimal flow would be:

• Filter geoms by bbox (only keep the geom's whose bbox's intersect with the radius circle geometry)
• Of those geoms remaining, union
• Of those unioned geometries, get the closet point