[WIP] Add NCRestart reader

[IAlibay]

Fixes #3940 (probably some others too)

This is a slightly cleaned up version of some mockup code I had knocking about on some branch a few years back. It needs cleaning up but I'm opening it as WIP to force myself to do so.

TODO:

• [ ] Generalise NCReader checks so to staticmethods which can just be used by the NCRestart reader
  • [ ] Make scale_factor checking a staticmethod
  • [ ] Switch to using NCReader's verify units
  • [ ] Switch to using _get_var_and_scale
• [ ] Set dimensions properly (currently using old version of doing things)
• [ ] Modernize testing code

Changes made in this Pull Request:

PR Checklist

• [ ] Tests?
• [ ] Docs?
• [ ] CHANGELOG updated?
• [ ] Issue raised/referenced?