[OCF-Wide] Unravel datamodel from OCF's repos

[JackKelly]

At the "Data Engineering Big Ideas" meeting on 5th March 2024, we discussed that nowcasting_datamodel has "tendrils" that reach deep into many of OCF's repos. It would be good to consider how to decouple datamodel from the rest of the code.

Related:

• Data eng big ideas spreadsheet - associates concerns with challenges.
• Data Engineering Big Ideas In-Person Meeting - 5th March 2024

[peterdudfield]

Have removed this from satip, good started for this issue