New spec for ObjectiveWMM

ObjectiveWMM is a simple repackaging for iOS of the C-language World Magnetic
Model published by the United States' National Geospatial-Intelligence Agency
(NGA) and the United Kingdom's Defence Geographic Centre (DGC).  WMM is
primarily useful to be able to determine the magnetic declination for a
given location on a given date, required in order to convert between
headings relative to true north and magnetic north.