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         Task to "refine" the imaging process.

     The  zeroth  order  in imaging is to compute a single gridding and beam
    for all channels, at the center frequency: this is the standard  way  of
    making imaging. Strictly speaking, it produces an angular scale inverse-
    ly proportional to frequency. This is obtained by setting ONEBEAM$ to NO
    (and BCOL$ = 0) in the imaging tasks.
     The  infinite  order  is  to  compute one beam per channel, obtained by
    stting ONEBEAM$ to YES, but it is a costly process as gridding is to  be
    redone for each channel.

      This  task is here for the first order correction of the angular scale
    varying with frequency, just making a linear interpolation  between  im-
    ages  computed with two different gridding because of the angular scale.
    The images (or beams) are assumed to be computed for the frequencies  of
    the 2 extreme channels.

      These  files  (.lmv  and  .beam)  can be prepared by running twice the
    tasks UV_MAP or UV_INVERT with parameter BCOL$ set to 1 and  the  number
    of channels respectively on each execution.

Gildas manager 2018-03-23