Setting human population density at setup

[smitdave]

Issue

Something awkward about the function make_xds_object() is that it requires both residence and HPop and it checks that length(HPop) == length(residence).

It also creates a special case of F_H.xds_object()

Proposed Solution

Since make_xds_object() is supposed to return a template, I think it's probably sensible to

• set HPop=rep(1, length(residence) to for make_xds_object()
• require a call change_HPop from within xds_setup()

This would keep with the spirit of make_xds_object() and thenF_H.xds_obj() would not be required. It is defined only for use in make_xds_object()

Discussion

I will probably get around to making this change sooner or later, if no one has any specific feedback.