Don't index on the active attribute

[smjenness]

We use dat$attr$active now only as a placeholder mainly to count the population size. But through some historical coding practices, related to the fact that we allow inactive (aka dead) people to stay on the network in the broader EpiModel framework, we use the variable for indexing vectors. That can and should be removed from any indexes. Make sure to pre/post test this to ensure no negative consequences.