Add igenomes dedicated utils

[maxulysse]

          I haven't touched igenomes - you still need to use `WorkflowMain.getGenomeAttribute` for now.

Originally posted by @adamrtalbot in nf-core/tools#2430 (comment)

[adamrtalbot]

I've done zero research on this, but essentially we would need to populate the parameters with the igenomes details prior to running. I assume this needs to be done before running any parameter validation etc. I'm not certain how parameters are propagated in Nextflow and if they must occur outside a workflow scope etc.

Currently this is done in WorkflowMain as a groovy script but if parameters are preserved this could be moved over to a Nextflow function.