P001Modelish protocol

Created P001Modelish protocol in order to make calls to isInvalid and isUnset easier on objects that could be either a model or a surrogate (via `id<AModelish>` types).

• Created P001Modelish with BOOL invalid and unset properties.
• P001Model and P001ModelSurrogate both now adhere to P001Modelish.
• The InchCoordish, InchLocationish, P_001P001ModelTestsModelish, and P_001P001ModelTestsModelWithoutDeallocish protocols all now have P001Modelish as a superprotocol.
• Updated project file with added P001Modelish.h.