Added RealitySpace and init methods to RealityLengthUnit

• Added a RealitySpace property (named space) to RealityLengthUnit.
	» Currently it's default-initialized and released without any ability to modify or override (since currently it isn't modifiable, but that may be changing soon).
• Added initWithType: creation method to RealityLengthUnit, which takes a RealityLengthUnitType and uses that as the unit's type, setting _unset as necessary.
	» “WithType” instead of “UsingType” because “with…” is more traditional for Obj-C as is the behavior here: we're just copying the bits to our own ivar.  “Using…” should be only for claiming shared or sole ownership of an P001Model.
• Publicized init method in RealityLengthUnit.
	• init now piggy-backs off of initWithType:

Minor:
• Changed the format of a “for typos” comment in RealityLengthUnitTests to make what it refers to more clearer.
Merge branch 'adhoc/travis-build-test' into feature/unit-models