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/driver/driver.py

@@ -29,7 +29,9 @@ class CXGDriver(metaclass=ABCMeta):
         return {
             "layout": None,
             "max_category_items": None,
-            "diffexp_lfc_cutoff": None
+            "diffexp_lfc_cutoff": None,
+            "disable_diffexp": False,
+            "diffexp_may_be_slow": False
         }
 
     @property