Minor: Using more `@(…)` boxing

For simple double-to-NSNumber conversions, there's no reason why I shouldn't be using the newer Obj-C `@(…)` boxing syntax.  So now I am (in InchCoord).

Also reduced setter lines from two (into intermediate ivar) to one (return directly).  Boxing the return of a getter is a simple and reliable operation, and doesn't need additional complexity for safety checks or breakpointing.
LengthUnit generalized to be Space-indeterminate

• `space` property's type is just P001Model (or id<P001Modelish>) now.
	» Since the space is just used as reference point and never actually used for anything, it can really be any P001Model, as long as that model shows equivalence to the space property of other things in the same space.
• `type` property removed; types will have to be done a different way for this to be space-independent.
	» Each type should probably be just an instance (singleton) relative to other instances (e.g. an inch unit instance is in the same space yet is 1/12 the size of a foot unit instance).
• LengthUnitType enum removed.
Minor: Fixed typo in comment for stringof() macro

“identifier”, not “indentifier”
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.