Change installation of software + add testing for singularity

[maxulysse]

As discussed in #30
I had to undo some changes to install Singularity only when needed, but I tried to improve them as well ;-)

[maxulysse]

By the way tests are of course working (at least on my fork)

[apeltzer]

Looks good to me! I like testing Singularity too, but agree with @ewels that it shouldn't really break since we're always using conda for both singularity and docker anyways. Still nice to test things!

[ewels]

What do you guys think about adding a subcommand to nf-core tools to install stuff? Maybe it’s overkill, but I was just thinking that it would avoid having to add scripts like this to every pipeline (and give a single location to update).

Could even be clever and auto-detect platform and stuff. Or just bail if not Linux.

[apeltzer]

Love that idea! Yes, that should keep our pipelines clean from install.sh scripts ;-) And Sarek might be able to use that too ;-)

[ewels]

Closing this now, as we will build functionality into nf-core tools if anything.