Compare timing of umap to tsne, pca

[seaaan]

Compare on datasets of a range of sizes (see arxiv paper for examples) using:

• Rtsne with PCA first
• Rtsne with no PCA first
• PCA alone
• Others?

https://twitter.com/jimhester_/status/996063591433416704 for possible timing package

This could be written as a vignette or something else