Some methods like UMAP crash when *n* <= 10

[Fil]

I took UMAP as an example, but note that many methods crash or return garbage on small datasets. We could probably limit the number of neighbors to n-1, etc?

[saehm]

I think i fixed the issue with "number of neighbors" > n - 1, but LTSA, for example, produces NaN's for small dataset. I have to look into deeper.