UV_CHECK BEAM_RANGES

      UV_CHECK BEAMS returns a variable named BEAM_RANGES[3,Nbeam] where
      Nbeam is the number of different synthesized beams that can be
      required by the UV data (based on the visibility weights).

      BEAM_RANGES[1,i]  is the first channel of Beam number i
      BEAM_RANGES[2,i]  is the last channel of Beam number i
      BEAM_RANGES[3,i]  is the total weight of visibilities of Beam number i

      BEAM_RANGES is later used by UV_PREVIEW to discard edge channels
      that often do not have the same weights as the bulk of channels.

      Variable NBEAM_RANGES indicates the number of beam ranges. It is -1
      if UV_CHECK has not yet been used.

      Most automatic imaging processes will assume BEAM_RANGES is relatively
      simple (2 edges and a dominant main range) and simply discard the edges
      if any, or complain if is too complex.

      For complex datasets with NBEAM_RANGES greater than 2 or 3,
      BEAM_RANGES could be used to segment the imaging and deconvolution
      process into ranges having a common beam. Setting BEAM_STEP
      to 1 is another possibility, but that creates 1 beam per channel,
      thus doubling the used space.