"fdb-utils list" returns 0.0 instead of 0 for levelist

Nobody has claimed this yet.

See the difference in outputs:
```console
$ fdb-axes param=500166,class=od,expver=0001
IndexAxis[(...),levelist=(0,0.005,0.01,0.02,0.03,0.06,0.09,0.1,0.18,0.27,0.54,0.81,1.62,14.58,2.43,4.86,7.29),(...)]
```
```
$ fdb-utils list --filter param=500166
(...)
levelist: [0.0, 0.005, 0.02, 0.06, 0.18, 0.54, 1.62, 4.86, 14.58]
(...)
```

This is relevant since the index stored in fdb is actually `0` and not `0.0`. When requesting through polytope with feature extraction, `levelist=[0.0]` does **not** work, it must be `levelist=[0]`. Although for some reason, with a full-field request also `0.0` works.

Start by reproducing the discrepancy with `fdb-utils list --filter param=500166` and compare it with the stored values shown by `fdb-axes`. Trace the list command's levelist formatting and its existing tests, if any. Done means integer-valued levels are displayed as `0` rather than `0.0` while decimal levels retain their precision, with regression coverage for this case.

Written by the indexing model from the issue text.