Skip creating merged objects if there are too many samples

[allyhawkins]

We have not been successful in completing the merged workflow when there are > 100 libraries being merged into a single object. Because of this we probably want to add a check for the total number of libraries being merged and if it's > 100 we skip creating the object and emit a warning message about how that object won't be created, similar to what we do for multiplexed samples.

Something similar to this but for total number of libraries:

scpca-nf/merge.nf

Lines 175 to 179 in f215046

	filtered_libraries_ch.multiplexed
	.unique{ it.project_id }
	.subscribe{
	log.warn("Not merging ${it.project_id} because it contains multiplexed libraries.")
	}

[allyhawkins]

Closed by #777