def each_sample(list = nil)
      @header.sample_subset_index(list).each { |i|
        yield VcfSample::Sample.new(self,sample_by_index(i))
      }
    end
def each_sample(list = nil)
      @header.sample_subset_index(list).each { |i|
        yield VcfSample::Sample.new(self,sample_by_index(i))
      }
    end
Walk the samples. list contains an Array of int (the index)