Add a vignette for life_table() and burden_le()

[ivanhanigan]

Something like this. I have been working on a demo.
I can plan to add this in the next few weeks.

## Set up scenario: use HRAPIE recommended CRF 
rr_use <- data.frame(est_type = c("RRper10", "RRper10_lci", "RRper10_uci"),
                     est = c(1.062, 1.040, 1.083))
rr_use
## estimate for change in exposure of
deltaX = 3

## Create demog data
demog_data = data.frame(age = dat$start_age,
                            population = dat$pop,
                            deaths = dat$death)
demog_data

## Calculate for RR and 95CIs
for(est_type in rr_use$est_type){
    ##
    rr <- rr_use[rr_use$est_type == est_type, "est"]
    print(est_type)
    
## Calculate life table
le = burden_le(demog_data,
               pm_concentration = deltaX,
               RR = rr)
## le

## Calculate yll from lifetable
impacted_le <- le[["impacted"]][, "ex"]
## impacted_le

an <- burden_an(demog_data,
                pm_concentration = deltaX,
                RR = rr)
yll <- burden_yll(an, impacted_le)