Adding Dim3 class

This adds a dim3 class for gpu code and a convenience class decorator for lang.CppClass

src/pystencilssfg/extensions/gpu.py

@@ -4,8 +4,9 @@ from pystencilssfg import lang
 def dim3class(gpu_runtime_header: str, *, cls_name: str = "dim3"):
     """
     >>> dim3 = dim3class("<hip/hip_runtime.h>")
-    >>> dim3.ctor(64, 1, 1)
-    'dim3{64, 1, 1}'
+    >>> dim3().ctor(64, 1, 1)
+    dim3{64, 1, 1}
+
     Args:
         gpu_runtime_header: String with the name of the gpu runtime header
         cls_name: String with the acutal name (default "dim3")