tidy data and content/tables

[hardingnj]

Reorganise of repo structure.

data contains some files that should be in content/tables. The following need to happen before review:

• [x] Add readme describing what lives where
• [x] Adjust seq tech notebook to read data from new location
• [x] Adjust performance eval notebooks to write to correct locations
• [x] Adjust X MER notebook to write to new loc
• [x] Change accessibility notebook to write to new location

[review-notebook-app[bot]]

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[hardingnj]

NB: This PR is based from the split-methods branch, which hopefully should be merged soon.

[hardingnj]

Lots of moved files- but the changes are relatively small, just renames and reruns for the most part.

[hardingnj]

This is reasonably high priority for merging- otherwise we run the risk of getting into conflicts!

[leehart]

Deleted tidy_data_dir branch