Improvements for GPU code generation

- turned on restrict keyword by default (makes large difference on GPUs)
- smarter block indexing: changing block size depending on domain size
  Example: previously there where (1,1,1) blocks when requested
  block size was (64, 1, 1) and domain size (1, 512, 512), now the
  block size is changed automatically to (1, 64, 1) in this case
- added __lauch_bounds__ to kernels to allow better optimizations from
  the CUDA compiler

pystencils_walberla/templates/GpuPackInfo.tmpl.h

@@ -13,6 +13,13 @@
 #define FUNC_PREFIX __global__
 {%- endif %}
 
+#ifdef __GNUC__
+#define RESTRICT __restrict__
+#elif _MSC_VER
+#define RESTRICT __restrict
+#else
+#define RESTRICT
+#endif
 
 namespace walberla {
 namespace {{namespace}} {