Update gis.py

small simplifications

oggm/core/gis.py

@@ -334,15 +334,15 @@ def define_glacier_region(gdir, entity=None, source=None):
     # Open DEM
     # We test DEM availability for glacier only (maps can grow big)
     if isinstance(source, list):  # when multiple sources are provided, try them sequentially
-        for i, src in enumerate(source):
+        for src in source:
             source_exists = is_dem_source_available(src, *gdir.extent_ll)
             if source_exists:
-                source = source[i]  # pick the first source which exists
+                source = src  # pick the first source which exists
                 break
     else:
         source_exists = is_dem_source_available(source, *gdir.extent_ll)
 
-    if source_exists is False:
+    if not source_exists:
         raise InvalidWorkflowError(f'Source: {source} is not available for '
                                    f'glacier {gdir.rgi_id} with border '
                                    f"{cfg.PARAMS['border']}")