Add point retention multiplier (#179)

* Add an option to retain N times as many points as usual at each zoom

* Tests for point multipler with specified and guessed maxzooms

* Work in progress on inverse spatial ordering

* Fix inverse spatial feature order

* --reorder was depending on a feature index that wasn't being preserved

* Separate ordering by feature_minzoom from ordering inverse-spatially

* Add a test for the inverse spatial ordering

* Store the basezoom/droprate/multiplier decisions in tileset metadata

* Progress on adding filters to tippecanoe-overzoom

* Type promotion for comparison

* Look up the attribute value for ordering

* Add test of thinning and ordering features

* Plumb tippecanoe_decisions metadata through pmtiles

* Be careful not to put infinities in JSON

* Fix accidental dropping in what is meant to preserve sparse points

* Start distinguishing true, false, and null in expressions

* Most of the type conversions

* Add boolean conversions

* Literals and conjunctions

* Add filtering to tippecanoe-overzoom

* Add a test of filtering in overzoom

* Fix boolean conjunctions

* Handle the combination of cluster size and filtering

* Rework dot dropping to reconcile density threshold and multiplier

* Revert "Rework dot dropping to reconcile density threshold and multiplier"

This reverts commit f253a66382.

* Retain points by multiplier within each tile, not in global probability

* Test that intends to verify that the multiplier is reversible

* Get the test to detect the discrepancy

* Mark the start of multiplier clusters with a magic attribute

* Add string-contains

* Add in and ni operators

* Revert "Look up the attribute value for ordering"

This reverts commit 56bc73e49a.

* Revert "Type promotion for comparison"

This reverts commit 6f3256f5af.

* Make number formatting in tippecanoe_decisions consistent

* Revert "Add a test for the inverse spatial ordering"

This reverts commit c8047de9ab.

* Revert "Separate ordering by feature_minzoom from ordering inverse-spatially"

This reverts commit 35b19a223c.

* Revert "Fix inverse spatial feature order"

This reverts commit 5978ecdb44.

* Revert "Work in progress on inverse spatial ordering"

This reverts commit fdf230f632.

* Somehow missed the tests associated with that last revert

* Round-robin assign attributes to partials from across the multiplier

* Count the multiplier separately in each layer

* Fix distribution of accumulated attribute across multiplier features

* Update changelog, version, and docs

* Add "is null" and "isnt null" expressions

* Update interpretation of FSL expressions to pass the tests

* Test to assert that polygons are unaffected by the multiplier

* Clean up and comment

* Remove accidental unused case

tests/muni/out/-Z11_-z13_-B15.json

@@ -10,6 +10,7 @@
 "minzoom": "11",
 "name": "tests/muni/out/-Z11_-z13_-B15.json.check.mbtiles",
 "strategies": "[{},{},{},{},{},{},{},{},{},{},{},{\"dropped_by_rate\":4727},{\"dropped_by_rate\":4643},{\"dropped_by_rate\":4210}]",
+"tippecanoe_decisions": "{\"basezoom\":15,\"droprate\":2.5,\"retain_points_multiplier\":1}",
 "type": "overlay",
 "version": "2"
 }, "features": [

tests/muni/out/-Z11_-z13_-Bf2000.json

@@ -10,6 +10,7 @@
 "minzoom": "11",
 "name": "tests/muni/out/-Z11_-z13_-Bf2000.json.check.mbtiles",
 "strategies": "[{},{},{},{},{},{},{},{},{},{},{},{\"dropped_by_rate\":2910},{},{}]",
+"tippecanoe_decisions": "{\"basezoom\":12,\"droprate\":2.5,\"retain_points_multiplier\":1}",
 "type": "overlay",
 "version": "2"
 }, "features": [

tests/muni/out/-Z11_-z13_-rf2000.json

@@ -10,6 +10,7 @@
 "minzoom": "11",
 "name": "tests/muni/out/-Z11_-z13_-rf2000.json.check.mbtiles",
 "strategies": "[{},{},{},{},{},{},{},{},{},{},{},{\"dropped_by_rate\":2542},{\"dropped_by_rate\":1547},{}]",
+"tippecanoe_decisions": "{\"basezoom\":13,\"droprate\":1.4510341140028376,\"retain_points_multiplier\":1}",
 "type": "overlay",
 "version": "2"
 }, "features": [

tests/muni/out/-Z11_-z13_-rf2000_-Bg.json

@@ -10,6 +10,7 @@
 "minzoom": "11",
 "name": "tests/muni/out/-Z11_-z13_-rf2000_-Bg.json.check.mbtiles",
 "strategies": "[{},{},{},{},{},{},{},{},{},{},{},{\"dropped_by_rate\":2546},{},{}]",
+"tippecanoe_decisions": "{\"basezoom\":12,\"droprate\":2.1055,\"retain_points_multiplier\":1}",
 "type": "overlay",
 "version": "2"
 }, "features": [

tests/muni/out/-Z11_-z13_-rf2000_-g2.json

@@ -10,6 +10,7 @@
 "minzoom": "11",
 "name": "tests/muni/out/-Z11_-z13_-rf2000_-g2.json.check.mbtiles",
 "strategies": "[{},{},{},{},{},{},{},{},{},{},{},{\"dropped_by_rate\":1349,\"dropped_by_gamma\":963},{\"dropped_by_rate\":747,\"dropped_by_gamma\":752},{\"dropped_by_gamma\":514}]",
+"tippecanoe_decisions": "{\"basezoom\":13,\"droprate\":1.176222768016331,\"retain_points_multiplier\":1}",
 "type": "overlay",
 "version": "2"
 }, "features": [

tests/muni/out/-r1_-K20.json

@@ -10,6 +10,7 @@
 "minzoom": "0",
 "name": "tests/muni/out/-r1_-K20.json.check.mbtiles",
 "strategies": "[{\"coalesced_as_needed\":4608},{\"coalesced_as_needed\":4608},{\"coalesced_as_needed\":4608},{\"coalesced_as_needed\":4608},{\"coalesced_as_needed\":4608},{\"coalesced_as_needed\":4607},{\"coalesced_as_needed\":4607},{\"coalesced_as_needed\":4606},{\"coalesced_as_needed\":6005},{\"coalesced_as_needed\":5393},{\"coalesced_as_needed\":5012},{\"coalesced_as_needed\":4727},{\"coalesced_as_needed\":4577},{\"coalesced_as_needed\":4003},{\"coalesced_as_needed\":3005}]",
+"tippecanoe_decisions": "{\"basezoom\":14,\"droprate\":1,\"retain_points_multiplier\":1}",
 "type": "overlay",
 "version": "2"
 }, "features": [