Explore Nyström sketches for PSD linops

[andres-fr]

Explore effectiveness of variations from the technique covered in Halko et al 2011, section 5.4

https://arxiv.org/pdf/0909.4061

[andres-fr]

Relevant (more recent) reference:
Tropp et al: Fixed-Rank Approximation of a Positive-Semidefinite Matrix from Streaming Data

https://arxiv.org/pdf/1706.05736

[andres-fr]

Also relevant reference:
Clarkson/Woodruff: Low-Rank PSD Approximation in Input-Sparsity Time
https://www.cs.cmu.edu/afs/cs/user/dwoodruf/www/cw17.pdf