make json dump understand numpy types

[ffont]

We can make the pymtg.io.json_dump function compatible with numpy types by defining our own JSONEncoder. See example to support numpy arrays, ints and floats:

class NumpyEncoder(json.JSONEncoder):
    def default(self, obj):
        if isinstance(obj, numpy.integer):
            return int(obj)
        elif isinstance(obj, numpy.floating):
            return float(obj)
        elif isinstance(obj, numpy.ndarray):
            return obj.tolist()
        return json.JSONEncoder.default(self, obj)

json.dump({}, open('filename.json', 'w'), cls=NumpyEncoder)