applyMagneticDeclination: Earth magnetic declination
Description
Instruments that use magnetic compasses to determine current direction need
to have corrections applied for magnetic declination, to get currents with
the y component oriented to geographic, not magnetic, north. Sometimes, and
for some instruments, the declination is specified when the instrument is
set up, so that the velocities as recorded are already. Other times, the
data need to be adjusted. This function is for the latter case.