Clean up and consolidate the preparation of source tables

The preparation of tables is very messy: I am using the `yadl` versions for
- company-employees
- movies
- us-accidents
- us-elections
On the other hand, `housing-prices`, `housing` (housing-metam) and `schools` are in their starting form. 

With the addition of depleted tables, and later of partially depleted tables, all preparation should be in the same spot for consistency. 