def write_sc(io, comment, *columns)
      io << '#%' << comment
      io << COLSEP << columns.join(COLSEP) unless columns.empty?
      io << "\n"
    end
def write_sc(io, comment, *columns)
      io << '#%' << comment
      io << COLSEP << columns.join(COLSEP) unless columns.empty?
      io << "\n"
    end
writes a checkm "structured comment"
 @param io [IO] the IO to write to
 @param comment [String] the comment
 @param columns [nil, Array<String>] columns to follow the initial comment