Split up monorepo into separate repositories

[martyngigg]

The current repository holds:

• platform infrastructure code
• ELT code:
  • pipelines-common package
  • ingest package
  • modelling package

There does not need to be any coupling between the platform and ELT code - in fact we don't want to encourage this! An easy split would be

• 1 repository for platform code
• 1 repository for ELT code

We could consider splitting up the ELT code further into repositories based around the data sources but this will likely make more sense when there is a clearer idea of data ownership. A single repository defining the data pipelines should be sufficient for now.