How can I reset a key value? When I change one, e.g.
p.offer(Keys.WALKING_SPEED, 0.1);
How can I reset it after that? I can’t find a function to get the default value. 
Try something like this:
Value<Double> walkingValue = p.getValue(Keys.WALKING_SPEED).get();
p.offer(walkingValue.set(walkingValue.getDefault()));