Make a vignette for incubation period analyses

[nickreich]

We have good code for this and it's just low-hanging fruit right now, when we find the time.

should include a table like this:

dist -- methods available for SE -- fitting methods available

L -- asymptotic, bootstrap -- optim, MCMC
W -- asymptotic, bootstrap -- optim, MCMC
G -- bootstrap -- optim, MCMC
E -- credible interval -- MCMC

and maybe some of the mathematics behind the asymptotic SE calculation.