refactor config to support different config options for datasets in different dataroots. (#1596)

This will give us the ability to specify different config options for
different dataroots.

the key of the dataroot dictionary is no longer the same as the dataroot_url.
Previously key==dataroot_url, and now those are separated.

Added an "is_multi_dataset" function to simplify logic where it branched on single vs multi.

Simplified the rest.py interface by no longer passing in the user annotations object, since
that can be retrieved from the dataset.

server/test/test_diffexp.py

@@ -15,8 +15,9 @@ class DiffExpTest(unittest.TestCase):
     """Tests the diffexp returns the expected results for one test case, using different
     adaptor types and different algorithms."""
 
-    def load_dataset(self, path, extra={}):
-        config = app_config(path, extra=extra)
+    def load_dataset(self, path, extra_server_config={}, extra_dataset_config={}):
+        config = app_config(path, extra_server_config=extra_server_config,
+                            extra_dataset_config=extra_dataset_config)
         loader = MatrixDataLoader(path)
         adaptor = loader.open(config)
         return adaptor
@@ -100,7 +101,7 @@ class DiffExpTest(unittest.TestCase):
             # create a sparse matrix
             h5adfile = os.path.join(dirname, "sparse.h5ad")
             create_test_h5ad(h5adfile, 2000, 2000, 10, apply_col_shift)
-            adaptor_anndata = self.load_dataset(h5adfile, extra=dict(embeddings__names=[]))
+            adaptor_anndata = self.load_dataset(h5adfile, extra_dataset_config=dict(embeddings__names=[]))
             adata = adaptor_anndata.data
 
             sparsename = os.path.join(dirname, "sparse.cxg")