Improve speed estimations.

Reasonable ground speed depends a lot on altitude, so plumb that
information through to the speed estimator.

Also adds calculations for ground speed based on desired mach. I don't
know if DCS is using the same formulas, but we should at least be
pretty close.

qt_ui/widgets/QTopPanel.py

@@ -122,6 +122,8 @@ class QTopPanel(QFrame):
     def negative_start_packages(self) -> List[Package]:
         packages = []
         for package in self.game_model.ato_model.ato.packages:
+            if not package.flights:
+                continue
             estimator = TotEstimator(package)
             for flight in package.flights:
                 if estimator.mission_start_time(flight) < 0: