Fix width-one iteration slices on GPU

When iteration_slice was specified, the GPU code generator used to handle integer slice components wrongly, setting start == stop instead of stop == start + 1. This MR fixes that and provides tests.

Also, this reveals a more fundamental problem with iteration slices on GPU; see #103

src/pystencils/gpu/kernelcreation.py

@@ -66,7 +66,8 @@ def create_cuda_kernel(assignments: NodeCollection, config: CreateKernelConfig):
         iteration_space = normalize_slice(iteration_slice, common_shape)
     else:
         iteration_space = normalize_slice(iteration_slice, common_shape)
-    iteration_space = tuple([s if isinstance(s, slice) else slice(s, s, 1) for s in iteration_space])
+    
+    iteration_space = tuple([s if isinstance(s, slice) else slice(s, s + 1, 1) for s in iteration_space])
 
     loop_counter_symbols = [LoopOverCoordinate.get_loop_counter_symbol(i) for i in range(len(iteration_space))]