Select the preferred backend module used for the spherical harmonic transforms in pyshtools.
select_preferred_backend([backend, nthreads])
backend : str, optional, default = 'ducc' : Name of the preferred backend, either 'shtools' or 'ducc'.
nthreads : int, optional, default = 1 : Number of threads to use for the 'ducc' backend. Setting this parameter to 0 will use as many threads as there are hardware threads on the system.