Display the rescale alt axis for the grouping step

Nobody has claimed this yet.

**Describe the change**
The use of slices *fluffiness* to derive the altitude rescaling value in the GROUPING step of ampycloud implies (possibly) a different scaling for distinct slice bundles.

Currently, these scaling values are not stored anywhere, such that from #86 onwards, the rescaled altitude axis can no longer be easily shown in the ampycloud diagnostic plots.

It would be great to correct this situation, but doing so is tough because:
1) the rescaling value (per slice bundle) is not stored anywhere (should it be ? Or should the plot re-compute it ?), and
2) we need to show "little" axis for each slice bundle (instead of one global one - no idea how to do that cleanly).

This is going to be a tough issue to fix.

Start by tracing the GROUPING step and the ampycloud diagnostic plots, using the rescaling introduced around #86 as context. Decide whether each slice bundle's scaling value should be stored or recomputed, and how to display a separate altitude axis for every bundle; done means the rescaled axes are shown correctly in diagnostic plots.

Written by the indexing model from the issue text.