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/cli/launch.py

@@ -12,7 +12,6 @@ import psutil
 from server.app.app import Server
 from server.app.util.errors import ScanpyFileError
 from server.app.util.utils import custom_format_warning
-from server.utils.constants import MODES
 from server.utils.utils import find_available_port
 
 
@@ -25,10 +24,10 @@ BIG_FILE_SIZE_THRESHOLD = 100 * 2**20  # 100MB
 @click.option(
     "--layout",
     "-l",
-    type=click.Choice(MODES),
-    default="umap",
+    default=[],
+    multiple=True,
     show_default=True,
-    help="Method for layout."
+    help="Layout name, eg, 'umap'."
 )
 @click.option(
     "--diffexp",