Fix problem with over-triggering on bearing rate change.
Fix problem with over-triggering on bearing rate change.

Flights with maximum recording frequency (every 1 second) were generating
a lot of noise in the bearing rate change calculation. Changed the code
to calculate bearing rate change to a fix that's at least 5 seconds in
the future, not to the next fix.

xiadz/igc_lib,xiadz/igc_lib