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/test/integration/tFlagTransfer.py

@@ -80,7 +80,7 @@ def test_chan_flag_transfer():
             "msin.useflag=false",
             f"msout={MSAVG}",
             "steps=[averager,preflagger]",
-            "averager.timestep=6",
+            "averager.timestep=3",
             "averager.freqstep=4",
             "preflagger.chan=[0]",
         ]