Undo/redo (#659)

* immutable crossfilter

* first cut at reducer refactor with cascade model

* add initial redo/undo implementation

* small optimization

* integrate expression with history

* add tests for new reducers and fix a couple of small initialization bugs

* treat tiny lasso selections as a clear

* better function name for clarity

* fix undo for differential expression

* remove logging

* fix regression due to bad merge

* cleanup and comments for clarity

* improve undoable configuration for flexibility

* fix stale comments

* remove debugging code from production build

* rename categoricalSelectionState

* rename file

* improve comments

client/src/reducers/resetCache.js

@@ -0,0 +1,28 @@
+/*
+Reducer which caches derived state to be used in a reset
+*/
+
+const ResetCacheReducer = (
+  state = {
+    world: null,
+    crossfilter: null
+  },
+  action,
+  nextSharedState
+) => {
+  switch (action.type) {
+    case "initial data load complete (universe exists)": {
+      const { world, crossfilter } = nextSharedState;
+      return {
+        ...state,
+        world,
+        crossfilter
+      };
+    }
+    default: {
+      return state;
+    }
+  }
+};
+
+export default ResetCacheReducer;