Add support for anndata backed mode (#943)

* initial cut at backed mode

* make flask multithreading conditional on debug flag

* update X access to support backed mode

* lint

* improve help message for backed mode

* fix tests

* add MatrixProxy to normalize supported matrix types

* add FAQ entry for --backed

* remove use of matrix.T

* clean up

* add ability to disable diffexp from CLI; add hueristic to detect likely slow diffexp calculation, and warn user

* fix tests

* do not print diffexp speed warning if diffexp is disabled

* tweak wording of diffexp speed messages

* add FAQ entry on --disable-diffexp

* revise heuristic for warning about slow diffexp

* use quick tooltip delay on diffexp button

client/src/globals.js

@@ -14,7 +14,9 @@ export const configDefaults = {
   features: {},
   displayNames: {},
   parameters: {
-    "max-category-items": 1000
+    "max-category-items": 1000,
+    "disable-diffexp": false,
+    "diffexp-may-be-slow": false
   },
   links: {}
 };