this function tests that all the remaining alive players have boarded the given vehicle
params: _vehicle
returns 0 if false, 1 if true eg:
[ExfilHeli] call MRH_fnc_AllAlivePlayersInVehicle
can be used in scripts and trigger eg waitUntil with {[_veh] call MRH_fnc_AllAlivePlayersInVehicle == 1}