Add automatic magnetic declination computation

[tvrusso]

When qDF was written, I copped out and made the magnetic declination a user-configurable setting that applies to all DF reports. This is inideal.

NOAA provides a library and data file that can be used to compute magnetic declination given a location, date, and altitude above ellipsoid (or even geoid, if I recall correctly). qDF should be adapted to use this library instead of the manual entry method.

The World Magnetic Model web site is http://www.ngdc.noaa.gov/geomag/WMM/DoDWMM.shtml