Test for SmoothingOperator
So far the tests only work on the RGSpace. A few problems however occurred:
-> smoothing on RGSpace with log_distances=True does not work -> smoothing on PowerSpace does not work, as the total flux of the Fields is not conserved (not even close, I will test it again as soon as Martin rewrote the code without python) -> HPSpace and GLSpace is also not tested as pyhealpix in not available on the test pipeline