It was introduced in d4a684cc (add isl_union_map_copy_map,
Fri Apr 8 09:47:04 2011 +0200) but left undocumented
and it was mainly intended to extract an isl_map from
isl_union_map lying in a single space, i.e., exactly
what the newly introduced isl_map_from_union_map does.

Signed-off-by: Sven Verdoolaege <skimo@kotnet.org>