def grouped_digits(digits)
      unless (digits.size % @base_scale) == 0
        raise "Invalid number of digits for group_digits (#{digits.size} is not a multiple of #{@base_scale})"
      end
      digits.each_slice(@base_scale).map{|group| scaled_digit(group)}
    end
def grouped_digits(digits)
      unless (digits.size % @base_scale) == 0
        raise "Invalid number of digits for group_digits (#{digits.size} is not a multiple of #{@base_scale})"
      end
      digits.each_slice(@base_scale).map{|group| scaled_digit(group)}
    end
Convert base `exponent_base` digits to base `scaled_base` digits
 the number of digits must be a multiple of base_scale