Make reference subpopulation configurable by the user

[kaitejohnson]

Goal

As implemented in #158, the reference subpopulation is set to be either the "auxiliary subpopulation" (individuals not captured by wastewater), or the largest site if the sum of the site populations exceeds the global population. It is possible that a user may want to set a site to be the 'reference' and we should allow them to do so... (though by default) it should stay as is.