InchLocation initialization methods

• Basic `init` initializer added to InchLocation's header.
• `initUsingX:y:` can take existing InchCoord(s) and retain them (either or both args can also be nil, in which case it `new`s them like normal).
	• `init` piggy-backs off of `initUsingX:y:` now.
• kNANPoint renamed to kUnsetValue.