Enumerate states & reduce dead removal

[cliffckerr]

~25% performance increase by:

• Using enumerate() in State (very small performance increase, <5%)
• By default only removing dead once every 10 timesteps (the rest of the performance increase, >20%)

[robynstuart]

if we implement this change, wouldn't we need to go through all the modules and make sure that we were always conditioning on people being alive before selecting them for e.g. pair formation, disease acquisition/progression, etc?

[RomeshA]

Neat! @robynstuart's issue is a good one although arguably, don't we already have to deal with that problem by allowing users to set remove_dead=False?

[cliffckerr]

It is a good point ... the baseline test didn't change, so it seems like it's working? But in any case (which seems to be happening now anyway, at least for SIR), it would be nice if "being dead" (boolean flag) was decoupled from having to resize the array (computationally expensive).