Corner cases for sil_score calculation in _gather_results()

[Hu-JIN]

There are cases where a cluster contains only 1 sample. In those cases, the sil_score for that signature is currently 0. It might be better to set the sil_score to 1 in those cases.

Example: I ran DenovoSig with mvNMF on simulated PCAWG Lung-AdenoCA data, with init=random and n_replicates=20. SBS3 is difficult to be discovered. It turns out that, in 19 out of the 20 cases, two SBS4's are extracted, while in the rest 1 case, an SBS3 is discovered. So in the final clustering result, there is a SBS3 cluster with 1 sample, and a SBS4 cluster with 39 samples.

This example also illustrates why it may not be good to force each cluster to contain one and only one sample from each replicate, as done in SigProfiler.

[Hu-JIN]

See eebdf3b