add --obs-names and --var-names CLI params (#371)

* add --obs-names and --var-names CLI params

* fix lint

* performance improvements in scanpy engine

* fix lint

* fix typo

* correctly handle sparse formats in diffexp

* fix diffexp and 1d slicing

* diffexp uses t-stat, not pval; clean up arg handling

* make _slice a static method

* revise scanpy tests to match new API

server/test/schema.json

@@ -6,6 +6,10 @@
   },
   "annotations": {
     "obs": [
+      {
+        "name": "name",
+        "type": "string"
+      },
       {
         "name": "n_genes",
         "type": "int32"
@@ -31,20 +35,16 @@
           "Dendritic cells",
           "Megakaryocytes"
         ]
-      },
-      {
-        "name": "name",
-        "type": "string"
       }
     ],
     "var": [
-      {
-        "name": "n_cells",
-        "type": "int32"
-      },
       {
         "name": "name",
         "type": "string"
+      },
+      {
+        "name": "n_cells",
+        "type": "int32"
       }
     ]
   }