Add Support for Loading/Saving Ancillary Variable to/from NetCDF

[lbdreyer]

Part of #3358

Modify the NetCDF loader/saver such that it supports AncillaryVariables.

---

Acceptance Criteria

• Added tests for loading a NetCDF file with an Ancillary Variable (that are passing) : see PI-3473: Netcdf loading ancillary variables #3556
• Added tests for saving a cube with an AncillaryVariable to NetCDF file (that are passing) : see PI-3473: Netcdf saving of ancillary variables #3550
• Additional related : PI-3473: Whatsnews relating to ancillary load + save. #3557

[pp-mo]

All the criteria are met, so this is all done
( all bar the feature-branch mergeback : #3704 )
Closing.