Select short aligner based on genome size

[sainsachiko]

Description of feature

Requirement:

• bwamem2 to be the default for short reads.
• Use short_aligner to switch to minimap2 for large genomes because indeed there's a maximum genome size bwamem2 can practically deal with. The switch could be implemented automatically in the pipeline when the genome is larger than say 5 Gbp.

[muffato]

I tried implementing that but it was not as straightforward as I thought. The sanger-tol sub-workflow expects a regular value, but I can only get the genome size through a channel.