Animal - excess reproductive mass

**Is your feature request related to a problem? Please describe.**
When a reproductive event occurs, reproductive mass is divided by the birth mass of the functional group to produce the number of offspring birthed by an average individual of that cohort. This often leaves a birth mass remainder. It is unclear what to do with this remainder.

**Describe the solution you'd like**
1) check madingley code to see what they do with the excess mass
2) it seems the excess mass could be discarded into a soil pool or maintained as reproductive mass
3) decide which approach is more appropriate