def scalar(magnitude = 1)
      terms.reduce(1) do |prod, term|
        prod * term.scalar(magnitude)
      end
    end
def scalar(magnitude = 1)
      terms.reduce(1) do |prod, term|
        prod * term.scalar(magnitude)
      end
    end
Get a scalar value for this unit.
 @param magnitude [Numeric] An optional magnitude on this unit's scale.
 @return [Numeric] A scalar value on a linear scale
 @api public