This PR 1) introduces a cruise_speed parameter to the AircraftType class
and uses it as an override for default TOT/Ground Speed calculations and
2) sets this for the AH64.
The reason for this change is that air starts with the Apache at a speed
>130kt seems to completely break the FCR, even if you subsequently slow
down. In the development branch, Liberation sets the Apache to travel at
168kt, so any player air starting won't be able to use their FCR and it
wouldn't be readily apparent as to why.
In the longer run this parameter may also be useful for other aircraft
e.g. to override the cruise speed to the most efficient etc.
We can always estimate a startup time now. Remove the nullability from
the result, cleanup the callsites, and eliminate
TotEstimator.mission_start_time since it no longer does anything useful.
