Minor: Suffixed InchLocation's `_x` and `_y` ivars with `Model`

Renamed InchLocation's `_x` ivar to `_yModel` and `_y` ivar to `_xModel` in order to confuse the fuck out of everyone…

	Just kidding!  `_x` _actually became `_xModel` and `_y` became `_yModel`.  Rationale is to draw a stronger distinction between “x” as a property (which could returns as a surrogate) and the actual stored “x model” (and likewise for “y”).