[assignGear] Engineer classes should receive explosiveSpecialist and possibly Engineer traits

[NikkoJT]

As per setUnitTrait, the Engineer trait is required to repair vehicles using a toolkit, and the ExplosiveSpecialist trait is required to defuse explosives using a toolkit. Currently, F3 Engineer (Demo) and Engineer (Mines) classes receive a toolkit, but not either of these traits, relying instead on the base unit type to provide the appropriate traits. This should be changed to add the traits in the assignGear class, allowing units with other base types to be assigned the engineer classes if necessary, and avoiding potential user-side trip-ups.