Add ConditionalFieldAccess (Field.Access after out-of-bounds check)

Adds a wrapper around a Field.Access that the access is only performed if a certain condition is met.

If I use this, I can safely perform calculations and adjoint calculations with ghost_layers=0 and obtain the correct gradients without separate boundary handling.

pystencils/backends/cbackend.py

@@ -426,6 +426,9 @@ class CustomSympyPrinter(CCodePrinter):
         )
         return code
 
+    def _print_ConditionalFieldAccess(self, node):
+        return self._print(sp.Piecewise((node.outofbounds_value, node.outofbounds_condition), (node.access, True)))
+
     _print_Max = C89CodePrinter._print_Max
     _print_Min = C89CodePrinter._print_Min