Setting startchan when updating column writes wrong channels

Created by: aroffringa

MWA datasets are large and need to be split up by channel and processed by running DPPP several times. This should in principle be possible with the msin.nchan and msin.startchan parameters, however due to a bug MSUpdater does not actually use offset the channels at the proper index. This is because the MSReader never sets startchan in the MSInfo object, only nchan.