WLS regression workflow

[afrubin]

We want to support the types of analyses that were possible in Enrich2, especially the weighted least squares regression and replicate combination using the random effects model.

This is likely to require some new plugins (like a random effects model plugin), but that's something we can use down the line to (for example) combine scores from individual barcodes into a single variant-level score.

The example workflow can target the existing Enrich2 example dataset, which already is downsampled to include a limited number of variants.