Fix CUDA support

pystencils/__init__.py

pystencils/boundaries/boundaryconditions.py

pystencils/boundaries/boundaryhandling.py

pystencils/cpu/kernelcreation.py

pystencils/datahandling/parallel_datahandling.py

pystencils/gpucuda/kernelcreation.py

pystencils/gpucuda/periodicity.py

pystencils/kernel_decorator.py

pystencils/kernelcreation.py

pystencils_tests/test_abs.py

pystencils_tests/test_buffer_gpu.py

pystencils_tests/test_cuda_known_functions.py

pystencils_tests/test_cudagpu.py

pystencils_tests/test_custom_backends.py

pystencils_tests/test_indexed_kernels.py

pystencils_tests/test_types.py