Understand the reason behind unassigned gender, age and location to last individuals in the population at the end of the simulation

[crangelsmith]

We observe that at the last step of the simulation, the individuals generated by the Immigration module do not get assigned an age, gender and ethnicity (this does happen correctly in the previous steps).

Here an example:

This should be linked to the immigration component in vivarium_population_spenser/population/immigration.py. I suspect that the data index doesn't get updated properly before the simulation ends.

Currently we ignore those individuals (that is the reason that we always select individuals with an assigned sex), but it would be useful to fix this problem.