Re-check all file format loaders for derived coordinate units

[pp-mo]

Following this comment

Since all Coords now default to units=None (translating to "unknown"),
whereas before the default was '1',
We should check creation of Aux Factories in all the file-loading code to ensure that sensible units are assigned wherever needed.

This also applies to iris-grib.

[pp-mo]

I checked this, and I think it has been handled everywhere relevant in both iris + iris-grib.

Notes:
It only applies in formats pp(+ff), netcdf and grib.
In all cases, factories are created programmatically from coords which were already created elsewhere.
We need to ensure that all those have the appropriate units.
That seems to be already all done :

• For the field-based formats, every coord is created with specific units.
• For netcdf, a constituent coord can only be assigned the default units ('unknown') if the netcdf var had no units attribute,
  but that would be a CF error in itself, so the file would be "bad".

The last case might have been missed previously, as we used to assume units of 1, but we don't need to support this anyway.
#3870 suggests a whatsnew note about this