add multi-layout support to back-end (#766)

* add multi-layout support to back-end

* remove obsolete code

* temporary code to apply heuristic choice of default layout

* fix tests

* update python tests

* more py lint

* PR review changes

* more PR lint

* PR lint

server/test/test_scanpy_engine_data_load.py

@@ -15,7 +15,7 @@ class DataLoadEngineTest(unittest.TestCase):
 
     def test_delayed_load_args(self):
         args = {
-            "layout": "tsne",
+            "layout": ["tsne"],
             "diffexp": "ttest",
             "max_category_items": 1000,
             "obs_names": "foo",