Add growth rate <-> doubling time functions

[erwald]

This closes #8.

I wasn't sure how to best fit it into the existing file structure, so definitely feel free to request changes.

Also, I chose to represent growth rates as fractions where e.g. 0.05 == +5%, but maybe it's more natural to let 1.05 == +5%? I could see it either way, anyway it's easy enough to change.

[erwald]

@peterhurford Thanks, I've pushed some changes, lmk how it looks!

[peterhurford]

Thanks!