private void build() {
    if (!built) {
      synchronized (this) {
        if (!built) {
          index.build();
          built = true;
        }
      }
    }
  }
Index to efficiently query points within a radius from a point.
<p>
Writes and reads are thread-safe, but all writes must occur before reads.

@param <T> the type of value associated with each point
planetiler-core/src/main/java/com/onthegomap/planetiler/geo/PointIndex.java