Update post-DADA2 plots for quantitative details and more direct comparison to pre-QC

[cjfields]

A few items:

1. The current merging heatmap is useful but doesn't convey the actual size of the fragments, but gives the maximum overlaps detected given the constraints of the settings. It would be useful to have both, and to have them scaled accordingly
2. The ASV distribution plot isn't scaled to ASV quantity across samples; it would also be good to have both versions