Support pre-calculated affinity matrix in ht.cluster.Spectral

[ClaudiaComito]

Feature functionality
Allow users to provide a pre-calculated affinity matrix when computing spectral clustering, as in scikit-learn SpectralClustering.

Also generally adapt the ht.Spectral API to scikit-learn.

Additional context
Related to parallelization efforts within the SCIMES project with @dcolombo

[ClaudiaComito]

We will work on this issue.

Keep in mind that the eigensolver is based on Lanczos at the moment (ht.linalg.lanczos), on small datasets it will be inefficient. A more efficient distributed eigensolver is in the making by @mrfh92

[github-actions]

Branch features/1740-Support_pre-calculated_affinity_matrix_in_ht_cluster_Spectral created!