Display the rescale alt axis for the grouping step

[fpavogt]

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.