ENH: cleanup List constructors (issue #725)

- add copy construct from UList

- remove copy construct from dissimilar types.

  This templated constructor was too generous in what it accepted.
  For the special cases where a copy constructor is required with
  a change in the data type, now use the createList factory method,
  which accepts a unary operator. Eg,

      auto scalars = scalarList::createList
      (
          labels,
          [](const label& val){ return 1.5*val; }
      );