introduce escort/defend/block/heal creature related missions

[scrom]

e.g.

• ensure person a gets to a location within x moves.
• kill an enemy before they reach a certain location.
• prevent a person dying
• heal a person

[scrom]

see issue #192 for a challenge with the "heal" missions

[scrom]

Now that creature paths are implemented, and we're looking at "fail" attributes, this is coming closer to reality.

For escort missions, it may be that you meet a creature in a particular location and their path is set to the destination when you arrive. This would need the similar "initial" attributes idea as issue #192 although with the added need to trigger those attributes when encountering a creature, not just when triggering the mission. (although this could be achieved with a parent/child mission setup with just initial destination attributes on the child.

[scrom]

the zombie mission includes preventing all friendly NPCs from dying. a fail check can now also be implemented to fail a mission if a creature is dead or an object is destroyed. It might be possible to fail a mission if a creature reaches a given location using a "currentLocationName" attribute. Would probably need to pass the mission with a timeout. E.g. keep x away from location y until z.

The code for the currentLoction Name would be:

        if (_currentLocation) {
            currentAttributes.currentLocationName = _currentLocation.getName();
        } else {
            currentAttributes.currentLocationName = null;
        };

In creature.getCurrentAttributes