Cleaning up the economic_curves library and converting it to a placeholder

[fumitoh]

The economic_curves library has been a mirror of https://github.com/open-source-modelling/insurance_python. However, the original repo has been updated since, and the lifelib mirror has been left untouched.

Rather than rendering stale notebooks, it makes more sense to replace the page with a link to the original repo along with a short introduction to the work done by open-source-modelling.

@open-source-modelling what do you think?

[open-source-modelling]

I am ok with both options. Either we do another integration, or just leave a link. I will have some free time in July. Maybe a short discussion on what adds the most value to lifelib?