Examples

   >IMARK CURR           mark current position
   >IMARK ALL            mark positions in current list
   >IMARK "SOURCE.LIS"   mark positions in given
                         text file
   >IMARK "SOURCE.LIS" SYMBOL=5 COLOUR=3
                         use different symbol and colour
   >IMARK SOURCE_LIST    mark positions in given
                         ASTERIX source list file
   >IMARK SOURCE_LIST    as aove but number the symbols
   >IMARK
    LIST file name> !        input positions as separate
    RA array of RA> ......   arrays of RA and DEC
    DEC array of DEC>.....