Bugfix: CUDA known functions have been overwritten

functions like absolute values could not be used in CUDA backend since
known_functions was replaced instead of updated

pystencils/backends/cuda_backend.py

@@ -59,7 +59,7 @@ class CudaSympyPrinter(CustomSympyPrinter):
 
     def __init__(self):
         super(CudaSympyPrinter, self).__init__()
-        self.known_functions = CUDA_KNOWN_FUNCTIONS
+        self.known_functions.update(CUDA_KNOWN_FUNCTIONS)
 
     def _print_TextureAccess(self, node):