brushable histogram brush selection responds to state updates (#670)

* add continuousSelection reducer and make histo respond to state changes

* correctly handle uninitialized state for brush move

* PR review tweaks

client/src/reducers/continuousSelection.js

@@ -0,0 +1,26 @@
+import { makeContinuousDimensionName } from "../util/nameCreators";
+
+const ContinuousSelection = (state = {}, action) => {
+  switch (action.type) {
+    case "reset World to eq Universe": {
+      return {};
+    }
+    case "continuous metadata histogram start":
+    case "continuous metadata histogram brush":
+    case "continuous metadata histogram end": {
+      const name = makeContinuousDimensionName(
+        action.continuousNamespace,
+        action.selection
+      );
+      return {
+        ...state,
+        [name]: action.range
+      };
+    }
+    default: {
+      return state;
+    }
+  }
+};
+
+export default ContinuousSelection;