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.

docs/schemas/FlagTransfer.yml

 description: >-
-  | Transfer flags from a lower-resolution MeasurementSet (MS).
-  | Note: this is an experimental feature.
+  Transfer flags from a lower-resolution MeasurementSet (MS). In case there is a mismatch between the input MS and `source_ms`, e.g., in terms of baselines, a filter can be applied. Hence, options of the Filter step are also valid. In that case we don't check whether the same stations are present, just that the number of baselines is the same.
+  Note: this is an experimental feature.
 inputs:
   type:
     type: string