Add back reduced_assign to sympyextensions interface

src/pystencils/sympyextensions/__init__.py

 from .astnodes import ConditionalFieldAccess
 from .typed_sympy import TypedSymbol, CastFunc
 from .pointers import mem_acc
+from .reduction import reduced_assign
 
 from .math import (
     prod,
@@ -33,6 +34,7 @@ from .math import (
 
 __all__ = [
     "ConditionalFieldAccess",
+    "reduced_assign",
     "TypedSymbol",
     "CastFunc",
     "mem_acc",