Expose, document and test isl_map_get_basic_map_list

Expose, document and test isl_map_get_basic_map_list

This matches isl_set_get_basic_set_list, which was exposed in d25e556
(add isl_set_get_basic_set_list, Wed Apr 29 13:41:45 2015 +0200) with
the following motivation:

"It is sometimes more convenient to manipulate a list of basic sets
rather than to iterate over them individually."

For isl maps the same motivation exists. Even though often higher-level
operations reduce the need to inspect the specific basic maps, there are
certain uses (e.g., inspecting how "complex" a map is or writing a custom
printer for a map) which require the inspection and sometimes sorting of
basic maps. Especially sorting is more easily expressed on a list of
basic maps. Hence, functionality to obtain a basic map list from a map
is exposed.

Signed-off-by: Tobias Grosser <tobias@grosser.es>
Signed-off-by: Sven Verdoolaege <sven.verdoolaege@gmail.com>