Thread safety

This MR introduces a simple check for thread safety in the kernel constrains check. If the assignments are not thread safe, optimizations like OpenMP or GPU should fail.

src/pystencils/boundaries/boundaryhandling.py

@@ -450,5 +450,6 @@ def create_boundary_kernel(field, index_field, stencil, boundary_functor, target
     dir_symbol = TypedSymbol("dir", np.int32)
     elements += [SympyAssignment(dir_symbol, index_field[0]('dir'))]
     elements += boundary_functor(field, direction_symbol=dir_symbol, index_field=index_field)
-    config = CreateKernelConfig(index_fields=[index_field], target=target, **kernel_creation_args)
+    config = CreateKernelConfig(index_fields=[index_field], target=target, skip_independence_check=True,
+                                **kernel_creation_args)
     return create_kernel(elements, config=config)