Fix the spectral fitter for Python deconvolution

The spectral fitter was unused so far. In https://gitlab.com/aroffringa/wsclean/-/merge_requests/510, a first example is added the makes use of the fitter. This MR makes sure that example works, in particular, the function would previoslu throw an exception when no fitting is requested and fit_and_evaluate() is called. This MR makes sure the fit_and_evalute() function return the input.