Fix sorting of annotations without begin and end

[jcklie]

Right now, all annotations are stored in a sorted list ordered by (begin, end). This enables us to use binary search when looking for covered annotations. There is one list for each annotation type. Some annotations do not have begin or end, these are currently hacked to be sortable by using (sys.maxsize, sys.maxsize) as a key. It should be made so that only annotations are put into sorted lists that have (begin, end), the others should use an ordinary list. These should also be selectable with cas.select.

[reckart]

Annotations that do not have begin/end are called "feature structures" ;)