What should happen to non-loaded coordinates etcetera?

[trexfeathers]

Sub-task of #5165

#5054 introduces the ability for the following objects to be ignored during NetCDF load, if there is a failure when trying to attach them to the loaded Cube:

• DimCoord
• AuxCoord
• CellMeasure
• AncillaryVariable

In these cases, the objects have been successfully created, but are discarded when they can't be added to the Cube. A big argument for more tolerant loading is the ability for users to fix problems in their data within Iris, so it would be more helpful to preserve the objects that can't be added, allowing the user to examine them and in some cases modify them to fix problems.

The priority of this is unknown, as non-loaded objects are new territory (rather than just raising an Exception). We can perhaps wait for user reports, and votes on this issue, before deciding if we should act.

It's also uncertain how we would achieve this. Possibilities:

• Convert the objects to Cubes and include them in the loaded CubeList.
• Enhance the concept of a CubeList so it can contain these other objects.
• Loading returns an extra Iterable/Mapping containing non-loaded objects.
• Save the unloaded objects to disk (might be an appropriate case for pickling).
• Make whatever behaviour optional via a context manager / loading parameter.

[rcomer]

Ages ago there was a feature request to load coordinates by themselves from NetCDF - #814.