All of the functions we calculated for our manuscript are included in this
function. To add additional functions, they can either be defined on the fly or, to
permanently include a new metric in all downstream simulations, it can be included
here. The function needs to be included with a name, and it must accept a metrics.input
as input. If the function needs additional elements not included in that input, then
the prepData function must also be revised. Currently MRD references the
non-abundance-weighted version of the function. This is simply so calcField knows
how to handle the results downstream. Reminder to modify that function if
abundance-weighted MRD is to be implemented here.