Iteration slices containing symbols are handled incorrectly by GPU codegen

The GPU code generator generates wrong thread-index guard code if iteration_slice contains user-defined symbols. In gpu/indexing.py, the shape value is substituted for any one symbol encountered in the slice, regardless of whether that symbol is actually related to the shape. See x-failing test in tests/test_sliced_iteration.py, line 60.

In my view, no efforts should be wasted trying to fix this bug within pystencils 1.x, since it is embedded so deep inside the system, and the use case is relatively niche. With pystencils 2.0, it will automatically be fixed.