add MDS as an algorithm option under the PCA tool

[Thomas Gilgenast]

the PCA tool currently covers many related dimensionality reduction algorithms under its umbrella - MDS sounds like it would fit right in with the rest of the family

[Thomas Gilgenast]

added mds option to pca

accessible by passing variant='mds' to compute_pca()

also accessible by passing -a mds to the lib5c pca tool

required a refactor to expose a new function make_pairwise_correlation_matrix_from_counts_matrix() in lib5c.algorithms.correlation

closes #12

→ <<cset 202a4aa753cc>>