- missed consistency in a few places.

- return nullptr (with automatic conversion to tmp) on failures
  instead of tmp<....>(nullptr), for cleaner coding.

INT: add support for an 'immovable' tmp pointer

- this idea is from openfoam.org, to allow creation of a tmp that is
  protected from having its memory reclaimed in field operations

ENH: tmp NewImmovable factory method, forwards as immovable/movable