ENH: code reduction in PackedList, PackedBoolList (issue #751)

- eliminate iterators from PackedList since they were unused, had
  lower performance than direct access and added unneeded complexity.

- eliminate auto-vivify for the PackedList '[] operator.
  The set() method provides any required auto-vivification and
  removing this ability from the '[]' operator allows for a lower
  when accessing the values. Replaced the previous cascade of iterators
  with simpler reference class.

PackedBoolList:

- (temporarily) eliminate logic and addition operators since
  these contained partially unclear semantics.

- the new test() method tests the value of a single bit position and
  returns a bool without any ambiguity caused by the return type
  (like the get() method), nor the const/non-const access (like
  operator[] has). The name corresponds to what std::bitset uses.

- more consistent use of PackedBoolList test(), set(), unset() methods
  for fewer operation and clearer code. Eg,

      if (list.test(index)) ...    |  if (list[index]) ...
      if (!list.test(index)) ...   |  if (list[index] == 0u) ...
      list.set(index);             |  list[index] = 1u;
      list.unset(index);           |  list[index] = 0u;

- deleted the operator=(const labelUList&) and replaced with a setMany()
  method for more clarity about the intended operation and to avoid any
  potential inadvertent behaviour.