Change bc_in%pop_density and lightning24 from patch-level to site-level

[samsrabin]

They're read from gridcell-level data and it doesn't seem like they're ever modified on a per-patch basis.

[glemieux]

Per discussion during the FATES software engineering meeting, this is not likely to be needed (or avaiable) at the patch-level in the near future. We can change this to be a site-level boundary condition.