Fix bitrot with how brighter-fatter kernels are handled, add option to use new gain values

Some things never worked (specifying the file as a config option), other may never have worked, and certainly don't now.

Make it so that new kernels can be used, and their use is logged clearly. Now that the new style kernels carry their gains with them, make sure that when using new-style kernels, a self-consistent gain is used.