Add, document and test isl_union_set_get_set_list
As with the previous commit, the same motivation applies:
It is sometimes more convenient to manipulate a list of sets rather than
to iterate over them individually.
Sorting elements (e.g., dependence distances, access offsets) according
to some priority function is one use case where lists are more
convenient to use than the foreach callback.
Signed-off-by: Tobias Grosser <tobias@grosser.es>
Signed-off-by: Sven Verdoolaege <sven.verdoolaege@gmail.com>