Dm 38 subset reads for single read plotting

[thekugelmeister]

Description

• Added a single source of randomness to utils.py, for use in the future. All future random effects should reference utils.rng, an instance of numpy.random.Generator`.
• Added read subset functionality to the read loaders in load_processed.py
• Percolated read subset functionality up to plot_read_browser

Known issues, to bring up during review:

• There is not currently a way to set the random seed for deterministic results.
• The interface for specifying random selection is clunky. Is it overkill? Any better ideas?
• KNOWN BUG: Requesting the plotting of a specific number of reads in plot_read_browser will not always result in plotting the same number of reads.
  • This is because of the preexisting read duplication bug, documented elsewhere.
  • I deemed this to be out of scope of this simple PR. Do you disagree?
• Consistent with the other existing sources of randomness, there is no test infrastructure to verify this functionality.

[OberonDixon]

I agree that the subset specification interface is a bit clunky but it seems workable for now, and easier to design better once we implement other subsampling in other parts of the package