Added .ngc support for sratools fasterqdump and prefetch

[alexblaessle]

PR checklist

Closes #XXX

• This comment contains a description of changes (with reason).
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[Midnighter]

Looks good. I think you can drop one level of conditionals, since a missing file will just end in the final else clause anyway, but it's certainly fine like this.

[apeltzer]

ping @maxulysse to the rescue :D - this will be useful for nf-core/fetchngs#238 too

[maxulysse]

ok, so as far as I can tell, the pair end tests are not working, but I'm fairly sure it's unrelated to your PR @alexblaessle, I'll have a look and try to come up with a separate PR to fix that so that we can merge your PR.
Expect a fix by the end of the day.

[maxulysse]

#4355 will solve the tests, sorry about that

[alexblaessle]

Thanks for all the help @apeltzer @maxulysse @Midnighter