[daip] no visibilities in image header

[Eric Greisen]

Do you also get data sets with visibilities?  I believe that FILLM
will sometimes create data sets to match particular observing modes
within the data that then turn out to have no visibilities with valid
data (i.e. only autocorrelations, pointing, all zero, flagged, etc)

If you did not get what you expected - in addition to these empty
files - let me know and I will get these data too and check with the
debugger.

Eric Greisen