[CWM] Create back-end class structure

[rogup]

What? Why?

This PR is for task 1 of #20

I've created the datasetService and class structure, ready for the remaining methods in the Dataset object to be implemented.

Note that this new datasetService isn't hooked up the API routes yet, since the new file structure isn't implemented in the test/data so some tests would fail. It made sense to wait after this mock data has been created (we're waiting on the spec from Colm and Marcel for this) before hooking them up, removing any unused implementation code from the old skeleton server, and adding UTs.

Checklist

• [x] Updated or added any necessary docs in docs/
• Can't add UTs yet, this is task 3 of #20
• [x] Checked that all automated tests pass

[rogup]

@wu-lee Please can you review these changes?