make world picklable

[sbarthelemy]

Here is an example of WorldObserver that tries to pickle world at each simulation timestep

#import cPickle as pickle
import pickle

class WorldPickler(WorldObserver):
    def __init__(self, world, file):
        self._file = open(file, 'wb')
        self._dumps = []
        self._world = world

    def init(self):
        pass

    def update(self, dt):
        world = self._world
        self._dumps.append(cPickle.dumps(world, protocol=-1))

    def finish(self):
        pickle.dump(self._dumps, self._file, protocol=-1)

However, it fails with this error:

pickle.PicklingError: Can't pickle <type 'instancemethod'>: it's not found as __builtin__.instancemethod