Introduction of structural ast nodes

Adds structural node category into AST, facilitating the distinction between expression nodes, structural nodes (e.g. statements, assignments, etc.) and other node types.

src/pystencils/backend/ast/structural.py

@@ -247,7 +247,7 @@ class PsLoop(PsStructuralNode):
             self._start.clone(),
             self._stop.clone(),
             self._step.clone(),
-            self._body.clone(),
+            self._body._clone_structural(),
         )
 
     def get_children(self) -> tuple[PsAstNode, ...]:
@@ -312,8 +312,8 @@ class PsConditional(PsStructuralNode):
     def _clone_structural(self) -> PsConditional:
         return PsConditional(
             self._condition.clone(),
-            self._branch_true.clone(),
-            self._branch_false.clone() if self._branch_false is not None else None,
+            self._branch_true._clone_structural(),
+            self._branch_false._clone_structural() if self._branch_false is not None else None,
         )
 
     def get_children(self) -> tuple[PsAstNode, ...]: