Update isolated_labels.py

[danielStrobl]

As mentioned in https://github.com/theislab/scib/issues/367, the isolated label metrics do not compute what they should. By calculating the individual silhouette scores and then taking the mean over only the cells of the cluster, this should be fixed.

[LuckyMD]

I guess you'll need to adapt the tests as well for this.

[codecov[bot]]

Codecov Report

Merging #368 (3aa8303) into main (1be0daa) will not change coverage. The diff coverage is 100.00%.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #368 +/- ## ======================================= Coverage 53.13% 53.13% ======================================= Files 22 22 Lines 1848 1848 ======================================= Hits 982 982 Misses 866 866 ``` | Flag | Coverage Δ | | |---|---|---| | unittests | `53.13% <100.00%> (ø)` | | Flags with carried forward coverage won't be shown. [Click here](https://docs.codecov.io/docs/carryforward-flags?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=theislab#carryforward-flags-in-the-pull-request-comment) to find out more. | [Impacted Files](https://codecov.io/gh/theislab/scib/pull/368?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=theislab) | Coverage Δ | | |---|---|---| | [scib/metrics/isolated\_labels.py](https://codecov.io/gh/theislab/scib/pull/368?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=theislab#diff-c2NpYi9tZXRyaWNzL2lzb2xhdGVkX2xhYmVscy5weQ==) | `94.33% <100.00%> (ø)` | |

[LuckyMD]

Please also adapt the perfect score test

[danielStrobl]

ah bit too fast

[LuckyMD]

@mumichae could you still review this please?