def as(unit)
      unit = UOM::Unit.for(unit.to_sym) if String === unit or Symbol === unit
      return self if @unit == unit
      Measurement.new(unit, @unit.as(quantity, unit))
    end
def as(unit)
      unit = UOM::Unit.for(unit.to_sym) if String === unit or Symbol === unit
      return self if @unit == unit
      Measurement.new(unit, @unit.as(quantity, unit))
    end
Returns a new Measurement which expresses this measurement as the given unit.