Feature: Add in Coale-McNeil model

[springcoil]

http://data.princeton.edu/pop509/ParametricSurvival.pdf

I don't see an obvious pdf in that documentation though. Perhaps I need to re-read it though.

[erikbern]

That PDF is great. I actually tried to find out more about Coale-McNeil but it sounds like it's only really been used to model marriage data. So maybe it's limited utility. I also unsuccessfully tried to track down the Swedish data set the PDF talked about

Some of the other ones would be great, in particular Gompertz