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

server/test/test_api.py

@@ -198,6 +198,7 @@ class EndPoints(unittest.TestCase):
         url = f"{URL_BASE}{endpoint}"
         params = {
             "mode": "topN",
+            "count": 10,
             "set1": {
                 "filter": {
                     "obs": {