New exit speed method

This implements the new exit speed method, allowing the users to enter values <= 0 which are interpreted as exit speed calculations based off of meanspeed, minus a percentage (ie. -0.05 would aim to exit with meanspeed*(1+(-0.05)).

I re-implemented the handling of the exitspeed for the beginning of the race so that it has a cutoff speed, and a non-zero forced_exit_speed which is temporarily enforced. It's very important that this value isn't zero, because otherwise the boat will immediately enter its final run, regardless of whether it really should or not. The forced_exit_speed value is high enough to force the boat into its normal manoeuvring, until within a few seconds it receives a properly calculated exitSpeed based off of meanspeed.