public void registerFeatureHandler(FeatureProcessor processor) {
    if (!caresAboutLayer(processor)) {
      return;
    }
    sourceElementProcessors.add(MultiExpression.entry(processor, processor.filter()));
    synchronized (sourceElementProcessors) {
      indexedSourceElementProcessors = null;
    }
  }
Call {@code processor} for every element.

@param processor handler that will process elements in that source
planetiler-core/src/main/java/com/onthegomap/planetiler/ForwardingProfile.java