ENH: add move reset and move assignment for tmp, tmpNrc

- improves similarity to autoPtr. Simplifies coding.

  Example,

    tmp<volScalarField> tfield;

    // sometime later...

    tfield.reset
    (
        volScalarField::New("myfield", mesh, dimensionedScalar(Zero))
    );

- as per tmp, disallow tmpNrc assignment from literal nullptr

- as per autoPtr, allow explicit test as bool (same as valid).