ENH: support writable reference for tmp (#1775)

- improves flexibility. Can tag a tmp as allowing non-const access to
  the reference and skip additional const_cast in following code. For
  example,

      tmp<volScalarField> tfld(nullptr);
      auto* ptr = getObjectPtr<volScalarField>("field");
      if (ptr)
      {
          tfld.ref(*ptr);
      }
      else
      {
          tfld.reset(volScalarField::New(...));
      }
      auto& fld = tfld.ref();

ENH: renamed tmpNrc to refPtr

- the name 'refPtr' (reference|pointer) should be easier to remember
  than tmpNrc (tmp, but non-ref-counted).

- provide tmpNrc typedef and header for code compatibility

NOTE

- in some places refPtr and tmp can be used instead of a
  std::reference_wrapper for handling external references.

  Unlike std::reference_wrapper, it can be default constructed
  (holding nothing), whereas reference_wrapper may need a dummy
  reference. However, the lifetime extension of references _may_ be
  better with reference_wrapper.