Update merged report UMAP sizing

[sjspielman]

In #636, we are adding merged UMAPs. Once we run some of the larger projects through, we'll want to revisit how we handle overall plot sizing. I've left some TODOs in the code for where we'll probably need to tweak some things.

From the #636 opening comment:

I took a first stab at some kind of sizing for the overall faceted plot, but we'll definitely want to look at all of them to choose the right calculations. If this seems not unreasonable as a first attempt, maybe let's leave it for now and figure it out later.

[sjspielman]

These TODO lines are the ones we'll need to revisit:

scpca-nf/templates/merged-report.rmd

Lines 309 to 319 in 3ac3070

	# Determine faceting layout based on n_libraries
	# TODO: This needs to be revisited when we run the merged objects through
	facet_ncol <- dplyr::case_when(
	n_libraries <= 20 ~ 4,
	n_libraries <= 50 ~ 6,
	n_libraries <= 80 ~ 8,
	.default = 10
	)
	# TODO: For now, 3 inches per UMAP row
	umap_height <- ceiling(n_libraries / facet_ncol) * 3

[allyhawkins]

Just noting that after looking at all the reports, everything seems to look good until you get up to 10 columns. The problem isn't the UMAP itself, it's the font size for the facets that need to be adjusted if you are using 10 columns.

This doesn't need to be fixed until we are able to produce a merged object with > 80 samples which is not the case yet.