Move all data to separate directory

[ElliottKasoar]

Summary

I haven't necessarily thought through all aspects of this, but I'm wondering if we ought to have a data directory independent to the code i.e.

ml_peg
  calcs
  analysis
  app

data
  calcs
  app

This would simplify git tracking, which currently has to ignore a huge amount of data, and potentially simplifies data that's shared between the calc and app e.g. phonon reference data, although I'm still not fully sure if this is easily solved.

We'd also need to check this isn't a problem for the mlipx tests, although I think it should be fine.

Example

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Alternatives

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Additional context

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