ENH: relocate protected List::size(label) to UList (issue #595)

- makes it accessible for containers that manage their own storage
  and derive directly from UList.

- DynamicList::min_size() method to access the corresponding
  SizeMin template parameter.

- ensure consistency in the reserve size for the constructor

      DynamicList<..> lst(N);

  now has identical sizing as

      DynamicList<..> lst();
      reserve(N);