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

server/app/rest_api/rest.py

@@ -69,7 +69,9 @@ class ConfigAPI(Resource):
                     "about-dataset": current_app.config["ABOUT_DATASET"]
                 },
                 "parameters": {
-                    "max-category-items": current_app.data.config["max_category_items"]
+                    "max-category-items": current_app.data.config["max_category_items"],
+                    "disable-diffexp": current_app.data.config["disable_diffexp"],
+                    "diffexp-may-be-slow": current_app.data.config["diffexp_may_be_slow"]
                 },
                 "library_versions": {
                     "cellxgene": cellxgene_version,