Check for "within" results from closest_point

[mapsam]

Refs: #36

This uses the internal boost::geometry::within check from closest_point by checking if the cp_info.distance == 0.0, which is only the case when a within statement returns true. This way we can assign the original query point as the resulting geometry and avoid double->integer->double math.

TODO

• clean up the conditionals since there's a bit of duplicate code right now
• benchmarks

cc @flippmoke @artemp

[mapsam]

closing in favor of #42