diffexp performance & UX improvements (#431)

* new diffexp REST API spec

* new diffexp REST API; faster diffexp and dataframe slicing

* first draft of fast diffexp

* convert variance calculation to two-pass method

* lint

* update front-end use of API

* fix typo in spec

* disable content compression

* catch index filter format errors

* clean up of dead code

* resolve PR review comments

client/src/actions/index.js

@@ -90,12 +90,16 @@ async function _doRequestExpressionData(dispatch, getState, genes) {
   const state = getState();
   const { universe } = state.controls;
   /* preload data already in cache */
-  let expressionData = _.transform(genes, (expData, g) => {
-    const data = kvCache.get(universe.varDataCache, g);
-    if (data) {
-      expData[g] = data;
-    }
-  }); // --> { gene: data }
+  let expressionData = _.transform(
+    genes,
+    (expData, g) => {
+      const data = kvCache.get(universe.varDataCache, g);
+      if (data) {
+        expData[g] = data;
+      }
+    },
+    {}
+  ); // --> { gene: data }
   /* make a list of genes for which we do not have data */
   const genesToFetch = _.filter(genes, g => expressionData[g] === undefined);
 
@@ -119,7 +123,6 @@ async function _doRequestExpressionData(dispatch, getState, genes) {
           }),
           headers: new Headers({
             accept: "application/json",
-            "Accept-Encoding": "gzip, deflate, br",
             "Content-Type": "application/json"
           })
         }
@@ -239,7 +242,6 @@ const requestDifferentialExpression = (set1, set2, num_genes = 10) => async (
         method: "POST",
         headers: new Headers({
           Accept: "application/json",
-          "Accept-Encoding": "gzip, deflate, br",
           "Content-Type": "application/json"
         }),
         body: JSON.stringify({