allow use of .gz when specifying fast and gtf

[ChristopherMancuso]

Description of feature

I was trying to run the scrnaseq pipeline specifying a fasta and gtf file. I downloaded from ensembl but they were gunzipped.

I searched Slack and found the same problem I was having and this answer that seems to have worked for me too https://nfcore.slack.com/archives/CHN5BV5DW/p1706103433405269

I was wondering if it would be possible to allow the use of .gz files like this? I have used the rnaseq pipeline and it allows for the use of gunzipped files. Thanks for this amazing resource!