Switch to core with fixed PlIteratorUtils

Signed-off-by: Raphaël Barrois <raphael.barrois@polytechnique.org>

diff --git a/core b/core
index ef815e7..a797e4f 160000 (submodule)
--- a/core
+++ b/core
@@ -1 +1 @@
-Subproject commit ef815e754cee390b7503d848cf753f0cef328d6b
+Subproject commit a797e4f7daf5fbcc9b12f64c298e9aa4df0f0acc