bg_id shouldn't be required to combine frames

[jhennawi]

It looks as if both the comb_id and bkg_id fields need to be there in order for pypeit to recognize those fields in the PypeIt file. I think that is actually a bug. In principle if the user wants to only combine frames but not difference, then the bk_ids need not be there. I’m trying to satisfy the 2d coadding requirement in confluence. Being able to omit the bkg_id in the pypeit file would make the interface slightly cleaner.