GetIn: Allow getting in 'hostile' vehicles (module)

This is a default Arma problem.
You can't board a vehicle if it contains a enemy.
Probably makes sense in many situations, but can cause problems

A door gunner of resuce chopper, who teamkills on accident will become sideEnemy, and prevent friendlies from boarding.
Also problems in RP game modes with non-cable tied hostages, enemy defectors...

To reproduce:
put gunner into blackhawk, addRating -10000 to simulate team kill, 
the Arma get in action-menu options will disappear
GetIn menu will still show GetIn available, but nothing will happen on use

This adds a simple optional module to allow getInPosition to always work.
In many environments, you don't want infantry running up to your tank and hopping in; so default behavior is unchanged.

Possible Problems:
using the forced moveInX doesn't trigger the getIn event handler - taken care of for now
`_units` could be removed.