ENH: additional PtrList constructor and memory management method

- PtrList::release() method.

  Similar to autoPtr and unique_ptr and clearer in purpose than
  using set(i,nullptr)

- Construct from List of pointers, taking ownership.

  Useful when upgrading code. Eg,

     List<polyPatch*> oldList = ...;
     PtrList<polyPatch> newList(oldList);
     ...

BUG: incorrect resizing method names (PtrDynList) in previously unused code