def convert_to(other)
      return self if other.nil?

      case other
        when Unit
          return self if other == self
          target = other
        when String
          target = SIUnits::Unit.new(other.to_f)
        else
          raise ArgumentError, "Unknown target units"
      end
    end
def convert_to(other)
      return self if other.nil?

      case other
        when Unit
          return self if other == self
          target = other
        when String
          target = SIUnits::Unit.new(other.to_f)
        else
          raise ArgumentError, "Unknown target units"
      end
    end
Comparable units
 => Compare with scale of kinds
 convert to a specified unit string or to the same unit as another Unit