Change the sign of the phases in all visibility columns

    Keyword arguments:
    vis -- Name of input visibility file
        default: none; example='3C273XC1.ms'
    spwlist -- Select spectral window
        default: [] all spws will be conjugated; example: spw=[1,2]
    outputvis -- name of output visibility file
            default: 'conjugated_'+vis; example= 'conjugated.ms'
    overwrite -- Overwrite the outputvis if it exists
        default=False; example: overwrite=True


           conjugatevis(vis='NGC253.ms', spwlist=[0,1], outputvis='NGC253-conj.ms')

        Will conjugate all visibilities for spectral windows 0 and 1 and store the
    modified data in NGC253-conj.ms.