RAP-796 Add flag transfer step

This MR adds the new flagtransfer step.

docs/schemas/FlagTransfer.yml

@@ -9,8 +9,7 @@ inputs:
   type:
     type: string
     doc: Case-insensitive step type; must be 'flagtransfer'.
-    default: flagtransfer
   source_ms:
     type: string
     doc: >-
-      Low resolution MS from which to extract flags. These flags are subsequently transfered to the MS specified by ``msin``.
+      Low resolution MS from which to extract flags. This MS must be an averaged version of the main input MS.