Add --accumulate-attribute to tippecanoe-overzoom (#189)

* Starting to factor out attribute accumulation into its own file

* Continuing to factor out attribute accumulation

* Reduce duplicate code

* Plumbing the accumulate-attribute option around

* Call the attribute accumulator

* Test that accumulation works

* Add missing #includes

* Don't sort within individual multiplier clusters

Doing so throws off the spatial distribution of the low zooms

* Docs and changelog

* Add comments

tile.hpp

@@ -8,18 +8,9 @@
 #include <map>
 #include "mbtiles.hpp"
 #include "serial.hpp"
+#include "attribute.hpp"
 #include "jsonpull/jsonpull.h"
 
-enum attribute_op {
-	op_sum,
-	op_product,
-	op_mean,
-	op_concat,
-	op_comma,
-	op_max,
-	op_min,
-};
-
 struct atomic_strategy {
 	std::atomic<size_t> dropped_by_rate;
 	std::atomic<size_t> dropped_by_gamma;