Minor: Clean-up for InchCoordPlaceholder & InchCoord_double

• InchCoordPlaceholder's initPlaceholder now has an area where further initialization could be done, if necessary (also makes it clearer what's going on earlier).
• Added kInchCoord_doubleUnsetValue as a constant for NAN.
• InchCoord_double's and InchCoord_NSNumber's `init`s now piggy-back off of `initWithDouble:` and `initWithNSValue:`, respectively (though InchCoord_double#init still needs to set _unset to YES manually, since unset means a doubleValue of NAN but the opposite isn't necessarily true).
• More common condition first in InchCoord_double#nsValue and InchCoord_NSNumber's#doubleValue.