Clearcal

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Re-initializes the calibration for a visibility data set

      Clearcal reinitializes the calibration columns in a measurement set.
      Specificially, it will set the MODEL_DATA column to unity in total intensity
      and zero in polarization, and it will set the  CORRECTED_DATA column to the original
      (observed) DATA in the DATA column.  Use the field and spw parameters to select
      which data to initialize.  If the dataset does not yet have the scratch
      columns, they will be created and initilized for the whole dataset (field and spw
      ill be ignored).  

      Keyword arguments:
      vis -- Name of input visibility file
              default: none; example: vis='ngc5921.ms'

      field -- Select field using field id(s) or field name(s).
                 [run listobs to obtain the list id's or names]
              default: ''=all fields
              If field string is a non-negative integer, it is assumed a field index
                otherwise, it is assumed a field name
              field='0~2'; field ids 0,1,2
              field='0,4,5~7'; field ids 0,4,5,6,7
              field='3C286,3C295'; field named 3C286 adn 3C295
              field = '3,4C*'; field id 3, all names starting with 4C
      spw -- Select spectral window
              default: ''=all spectral windows and channels
              spw='0~2,4'; spectral windows 0,1,2,4 (all channels)
              spw='<2';  spectral windows less than 2 (i.e. 0,1)
              spw='0:5~61'; spw 0, channels 5 to 61
              spw='0,10,3:3~45'; spw 0,10 all channels, spw 3, channels 3 to 45.
              spw='0~2:2:6'; spw 0,1,2 with channels 2 through 6 in each.
              NB: Multiple channel ranges per spw are not supported in clearcal.