Minor fixes

I've implemented a few minor fixes in this MR; notably

1. I've added in the window_fftgram_dict option everywhere such that one may pass it around. I have also added the function util.get_window_tuple which unpacks the dictionary and puts it in proper format to be passed to scipy.signal.get_window. This now allows us to use tukey windows e.g.
2. I have added in option 1. also in util.window_factors, such that effectively delta_sigma_cut.WindowFactors is now obsolete. @alba.romero , could you please integrate this in your edits to the dsc? Thanks!
3. I've fixed small bugs in parameters and elsewhere which were breaking the pygwb_combine script. Thanks for catching these @kevin.turbang and Adrian!
4. I added the frequency_mask as a saved product of the baseline in all formats.

I also considered adding set_point_estimate methods to the OmegaSpectrumlike objects, but decided against it for now as this would require defining two spectra, one for the point estimate and one for sigma, in those objects...

for details please see the changes.