def coord(col, row=0)
      coordinates = parse_coord_args(col, row)
      self.col = coordinates[0]
      self.row = coordinates[1]
    end
shortcut to set the column, row position for this marker
@param col the column for the marker, a Cell object or a string reference like "B7"
or an Array.
@param row the row of the marker. This is ignored if the col parameter is a Cell or
String or Array.