add incidence/prevalence over time plots

percolations.py
- track number of new infections and total
infections at each time step using lists in
episim_age_time function
- incidence and prevalence output lists were
added to the data structures returned by the
function

age_perc_T_time.py
- dicts to store incidence and prevalence
by timestep for each sim
- added plots of new infections and total
infections over time for each simulation
that turns into an epidemic in order to
compare with OR over time
- need to figure out how to do average OR
at time = t for all simulations