clear differential expression reducer

client/src/reducers/differential.js

@@ -41,6 +41,13 @@ const Differential = (
         ...state,
         celllist2: action.data
       };
+    case "clear differential expression":
+      return {
+        ...state,
+        diffExp: null,
+        celllist1: null,
+        celllist2: null
+      }
     case "reset World to eq Universe":
     case "set World to current selection":
       return {