Stop applying things to the dead / unborn

[mmcleod89]

Two possible solutions:

1. Ignore dead / unborn by default in the dataframe wrappers
2. Remove dead / unborn people from the dataframe
   • This would improve efficiency by not wading through junk
   • Need to ask HIV team if they need to track them for some reason
   • Can potentially collect death statistics at each timestep and then remove those people
   • Can potentially add new people into the dataframe at some appropriate rate

[mmcleod89]

Presently needs tests