Add, document and test isl_union_map_get_map_list
As with the previous commit, the same motivation applies:
It is sometimes more convenient to manipulate a list of maps rather than
to iterate over them individually.
Sorting elements (e.g., dependences relations, memory accesses relations)
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>