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/components/geneExpression/expressionButtons.js

@@ -9,8 +9,8 @@ import CellSetButton from "./cellSetButtons";
 
 @connect(state => ({
   differential: state.differential,
-  world: state.controls.world,
-  crossfilter: state.controls.crossfilter
+  world: state.world,
+  crossfilter: state.crossfilter
 }))
 class Expression extends React.Component {
   constructor(props) {