Add download options

[rannick]

Description of feature

For fusioncatcher and starfusion, reduce building time and issues by having pre-built indexes on sourceforge

[apeltzer]

We will discuss with @nf-core/core if its a possibility to have the entire nf-core/rnafusion indices apart from the fusion-report (cosmic licence required atm) being added to the nf-core S3 bucket. We do this for some pipelines already, if people from core could give me a thumbs up / down / comment that would be appreciated 👍🏻