Revise rodent data formatting

[ChloeRN]

Following discussions on 05/05/23, we should make some changes and additions to rodent data formatting:

1. Sum rodent abundances corresponding to the fox year (autumn[t] + spring [t+1] instead of spring[t] + autumn[t])
2. Scale autumn and spring abundances BEFORE summing (this because relative abundances and catchability are not comparable among seasons)
3. Have two sets of covariates: one species-naive (voles = lemmings) and one weighting voles and lemmings before summing (this giving more weight to lemmings and at least partially offsetting their relatively lower catchability)

[ChloeRN]

For the final decisions on covariate structure, we will have to work with a combination of biological hypotheses and comparing effect size.

[ChloeRN]

Addressed via #27.