add flee() method

flee() takes all predators inot account, calculates escape vector as
average of vectors to each predator
all creatures now use the same update() method

aquireTarget() and overridden for individual behavior
chase() added as overrideable method for chasing a target-- seek() or
pursue()
names of predator and prey arrays in sub classes changed
TODO -- review name changes, make them more logical
setPlaceInFoodChain() sets predator/prey lists

predator and prey lists in Creature are set to the creature's type lists
Removed of update methods
Removed update() from Grass

Commented old update() in Creature