Model manipulate…s' primary block arg no longer “existing”

• Changed model manipulate… methods' primary block arg (the variable being manipulated, and the one being return from the block) to exclude the use of the term “existing” (e.g. “existingValue” is now just “value”).
	» This makes it cleaner to just modify the arg passed into the block and return it, which is fine.
	» Prefix notes: ”former” is used for additional block args that will need to be updated as a result of a change in the primary block arg; “existing” and “new” are used in contexts where pre- and post-change local vars need to be differentiated (such as a manipulate… implementation that uses other manipulate…(s)).
	• For value types, the primary arg is also now non-const, while non-primary args are still const.
	• Affected model classes: InchCoord, InchLocation
• Usages of model manipulate methods updated in:
	InchCoord.m, InchCoordProxyTests.m, InchLocation.m, InchLocationTests.m