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.