def unit_string
      use_dimension
      a = []
      a << Utils.num_inspect(@factor) if @factor!=1
      a += @dim.map do |k,d|
        if d==1
          k
        else
          "#{k}^#{d}"
        end
      end
      a.join(" ")
    end
def unit_string
      use_dimension
      a = []
      a << Utils.num_inspect(@factor) if @factor!=1
      a += @dim.map do |k,d|
        if d==1
          k
        else
          "#{k}^#{d}"
        end
      end
      a.join(" ")
    end
Make a string of this unit expressed in base units.
 @return [String]
 @example
   Phys::Unit["psi"].string_form #=> "(8896443230521/129032)*1e-04 kg m^-1 s^-2"