pystencils should also work when llvm and/or dot is not available

-> moved llvm and dot import in try-except block

backends/__init__.py

-from .llvm import generateLLVM
 from .cbackend import generateC
-from .dot import dotprint
+
+try:
+    from .dot import dotprint
+    from .llvm import generateLLVM
+except ImportError:
+    pass