adds script to generate synthetic data

[tomjemmett]

largely follows what is in the nhp_model repo's notebook, but adds this to the data extraction pipeline.

rather than extracting to the dev folder, it extracts to synth

closes The-Strategy-Unit/nhp_model#246

[tomjemmett]

need to handle the TODO before ready for review. We currently modify two HRG's to be called HRG1/HRG2, this is purely needed because that is what they are labelled as in the sample params

options:

1. change sample params to pick a valid HRG
2. remap all HRGs to HRG1..N,
3. push the remap list from the ip step into the object, so it can be picked up by inequalities (though, this forces a specific order for the steps to run in)

[yiwen-h]

Awesome! Thank you @tomjemmett - makes loads of sense.

Wonder what's the best way of documenting this for developers? Add to README or repo wiki page?