RAP-886 Add a Filter sub-step to FlagTransfer

LINC assumes that source_ms has fewer baselines than the target MS, which currently throws an error, since the step requires the source and target MSs contain the same number of baselines. This MR adds a Filter sub-step to facilitate these type of situations.

steps/FlagTransfer.cc

@@ -122,7 +122,8 @@ bool FlagTransfer::process(std::unique_ptr<DPBuffer> buffer) {
   std::vector<common::rownr_t> filtered_row_numbers =
       result_buffer->GetRowNumbers().tovector();
 
-  const common::rownr_t first_row_number = filtered_row_numbers[0];
+  const common::rownr_t first_row_number =
+      buffer->GetRowNumbers().tovector()[0];
   for (auto& row_number : filtered_row_numbers) {
     row_number -= first_row_number;
   }