def summary(indent_level=0)
      non_missing = size - count_values(*Daru::MISSING_VALUES)
      summary = '  =' * indent_level + "= #{name}" \
                "\n  n :#{size}" \
                "\n  non-missing:#{non_missing}"
      case type
      when :object
        summary << object_summary
      when :numeric
        summary << numeric_summary
      end
      summary.split("\n").join("\n" + '  ' * indent_level)
    end
Create a summary of the Vector
@param indent_level [Fixnum] indent level
@return [String] String containing the summary of the Vector
@example
dv = Daru::Vector.new [1, 2, 3]
puts dv.summary

# =
#   n :3
#   non-missing:3
#   median: 2
#   mean: 2.0000
#   std.dev.: 1.0000
#   std.err.: 0.5774
#   skew: 0.0000
#   kurtosis: -2.3333