STYLE: remove UList operator[] taking std::initializer_list

- unnecessary. Can deduce labelRange from the pair of labels.
  These are all the same:

      list[labelRange(18,3)] = 100;
      list[labelRange{18,3}] = 100;
      list[{18,3}] = 100;

  Removing the run-time handling of std::initializer_list in favour of
  compile-time deduction allows the future use of sliceRange as well.
  Eg,

     list[sliceRange{18,3,2}] = 100;
     list[{18,3,2}] = 100;