Fixed broken Iterator interface implementation in EnrichedActivity class

[davgothic]

The current implementation of the Iterator interface will stop looping over the activityData array as soon as a null value is found due to the following method:

public function valid()
{
    return (bool) $this->current();
}

I have replaced the implementation of the Iterator interface with the more suited IteratorAggregate.

[tbarbugli]

using the IteratorAggregate interface is indeed a much better approach. thank you for your contribution!