Added surrogate method to P001ModelSurrogate

Added a -surrogate method to P001ModelSurrogate, which:
	• if the model is still alive, asks it for a surrogate, and
	• if the model is dead, simply returns itself

The rationale for this is that without (as it has been), a call to surrogate on a P001ModelSurrogate would punch through to the model and return a new or the same surrogate.  However, once the model dies, then the method would dead-end and probably return nil (or just flat-out crash).  Providing the method is similar to providing methods for isInvalid and isUnset; they're a bit more efficient than what's already there, and extend the usage past model death.

• Added to P001Modelish so that all models and surrogates can use -surrogate equally.