ENH: add factory method readContents to IO containers

- useful when regular contents are to be read via an IOobject and
  returned.

  Eg,  dictionary propsDict(IOdictionary::readContents(dictIO));
  vs.  dictionary propsDict(static_cast<dictionary&&>(IOdictionary(dictIO)));

  Commonly these would have simply been constructed directly as the
  IO container:

  eg,  IOdictionary propsDict(dictIO);

  However, that style may not ensure proper move semantics for return
  types.

  Now,
  =====
      labelList decomp(labelIOList::readContents(io));
      ... something
      return decomp;
  =====

  Previously,
  =====
      labelIOList decomp(io);

      // Hope for the best...
      return decomp;

      // Or be explicit and ensure elision occurs...
      return labelList(std::move(static_cast<labelList&>(decomp)));
  =====

  Note:
       labelList list(labelIOList(io));

       looks like a good idea, but generally fails to compile