branching_ratios_for_multiple_reactions

[shimwell]

Attempting to help with https://github.com/openmc-dev/openmc/issues/2757

This PR changes the add_branching_ratios.py script so that multiple reaction types can be added. The two branching_ratio json files have also been changed so that the existing ratios are in a key called n,gamma and other keys can also be added. The json format is not more flexible to support other reactions that are not n,gamma

I have also made the same PR to my own adapted data repo which might be of interest as it runs a few tests