Return the specified backend module used for the spherical harmonic transforms in pyshtools.
module = backend_module([backend, nthreads])
backend : str, optional, default = preferred_backend() : 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.