`interp_coh` arg conformability

[timriffe]

TODO list:

• [x] ensure births cover needed years if given in args.
• [x] modularize birth download
• [x] check c1 c2 age ranges match
• [x] ensure nlx covers needed ages. Use OAnew arg on the fly to extend.
• [x] warn if intercensal span > 15 years
• [x] add verbose arg
• [ ] better px adjustment on the edges