Broke out InchCoord classes into separate files

• Moved InchCoord_double and InchCoord_NSValue class interfaces and implementations into InchCoord_double.h/.m and InchCoord_NSValue.h/.m, which are now included into InchCoord.m.
• Created an InchCoord_Common.inc for the (currently) one shared constant we need in both the subclasses and perhaps the main class.
	» As a sourcefile-included .inc, this generates the same data multiple times.  While this is fine for constants holding only a little data, it won't work for shared static vars.  If that need presents itself, this Common.inc will need to be converted to a Common.h/.m.
• Renamed `#pragma mark` headers to more simply describe the sections of each file.
	» Now “Internal Constants, Static Vars, Functions”, “Internal Model Interface”, “Model Implementation”, and “Surrogate Implementation”
	» I think I'm going to start using these for every file.  Even though I'm laying them all out the same and know myself what goes where, this makes them easier to grok fresh, and just separates things a bit better.
• Added the new files (InchCoord_double.h/.m, InchCoord_NSValue.h/.m, InchCoord_Common.inc) to the project file with appropriate target memberships.