Add one or more place entries for Mexico and Central America

[Megachile]

At this point I'm not sure what resolution we want to use for range south of the US (countries? just one big category for Mexico+Central America? Mexico as a separate category plus the rest of CA lumped together?) but it would be nice to have at least something to indicate range for galls we're adding outside the US/Canada. What would be really ideal is to allow galls in that place to be searched without even a host genus selected, since there are many fewer of them and host ID is much harder, but I imagine that's not worth the effort. Whether this information is reflected on range maps is not urgent IMO although now that the host range mapping is only done through the map interface I suppose there's not much choice there.