* rulesets/basic/mind/goals/common/trigger.py: Generic common trigger
goals, starting with a trigger base chase goal.
* rulesets/basic/mind/goals/animal/domestic.py: Trigger goals specific
to domesticated animals, starting with goals for pig herding.
* rulesets/basic/mind/goals/__init__.py: Add the two new files of
trigger goals.
* rulesets/mason/mind/PigMind.py, rulesets/mason/mind/SkelMind.py,
rulesets/mason/mind/WolfMind.py, rulesets/mason/mind/__init__.py:
Remove special mind classes for pig, skeleton and wolf, as it
should be possible to do it more cleanly with trigger goals.
* rulesets/mason/define_world.py: Add trigger goals for pig and
skeleton to replace the behavior that used to be in special purpose
mind classes.