update for spontaneous fission for branch_ratio()

This is a corollary to #523 for spontaneous fission in the branch ratio function.  Again for @katyhuff or @gidden since this is for cyclus or @crbates since it is a pretty hefty refactor of one of his functions.

**Bounty** - 2
exactly like above (`offsetof(...)` is in the `data_access` constructor)