All of the null models we calculated for our manuscript are included in this
function. To add additional null model functions, they can either be defined on the fly
or to permanently include a new model in all downstream simulations, it can be
included here. The function needs to be included with a name, and it must accept a
nulls.input object. If the function needs additional elements not included in that
input, then the prepNulls function must also be revised.