Add useValues hook

[axelboc]

Follow-up to #1963 to add a useValues hook.

The old useDatasetsValues and usePrefetchValues hooks were, in the end, quite specific to NeXus in that they accepted arrays of potentially undefined datasets, (Dataset | undefined)[] (for axes and auxiliaries). So I moved them to the nexus pack and renamed them useNxValues and usePrefetchNxValues respectively (see first commit).

This freed up the more generic useValues name for a hook that accepts a record of datasets (i.e. typically the one returned by useDatasets).

Each value in the record can also be an object with a dataset and selection key:

const { slice } = useValues({
  slice: { dataset: twoD, selection: '2:0' },
});

[loichuder]

That's so cool 👍