def dimension
      q = nil
      u = self.base.si_units
      SI_UNITS.each_pair do |dim, unit|
        unit = Measure.new(1.0, unit) unless unit.kind_of?(Measure)
        if unit.base.si_units == u
          q = dim
          break
        end
      end
      q
    end
def dimension
      q = nil
      u = self.base.si_units
      SI_UNITS.each_pair do |dim, unit|
        unit = Measure.new(1.0, unit) unless unit.kind_of?(Measure)
        if unit.base.si_units == u
          q = dim
          break
        end
      end
      q
    end
dimension (quantity)