Dynamically set groupTuple size param

[jdhayhurst]

Setting the groupTuple size parameter here:

gwas-sumstats-harmoniser/subworkflows/local/main_harm.nf

Line 13 in 98a573c

hm_files = harmonization.out.hm_by_chrom.map{it[0,2]}.groupTuple()

will mean that the next steps can start before other independent steps are finished. The trick is going to be dynamically setting this size. We can create a size map for each GCST with relative ease, but passing the value in here is not obvious. The closest doc I can find is here: https://github.com/nextflow-io/nextflow/pull/3283/files which elaborates on the groupKey method.