Use DataLocator consistently on backend (#1208)

* allow DataLocator to accept another locator as init param

* migrate to DataLocator

* migrate to DataLocator

* lint

* migrate to DataLocator

* add check for erroroneous use of remote path and annotations

* lint

* revert default data location - now back go CWD

* remove unused import

server/eb/app.py

@@ -45,7 +45,8 @@ try:
         obs_names=None,
         var_names=None,
         anndata_backed=False,
-        disable_diffexp=False)
+        disable_diffexp=False,
+    )
 
     matrix_data_cache_manager = MatrixDataCacheManager()
     annotations = None