Split JCasIterable into iterable and iterator parts

The JCasIterable is an Iterable and an Iterator at the same time. The iterator() method returns just "this". This goes against the expectation that an Iterable should return a fresh iterator when iterator() is called.

JCasIterable should therefore be split. The JCasIterable should only be creatable with descriptions.

Calling iterator should create a new JCasIterator which then is based on concrete reader and AE instances.