Bit more on Spack, fix in tutorial

[brenthuisman]

• Bit more on Spack and how or why to use it, plus an instruction that is required on some systems.
• Fix in network_ring tutorial

[schmitts]

Thanks. We can also add a bit on how to build catalogues which needs the prefix spack build-env arbor, e.g.:

spack build-env arbor arbor-build-catalogue custom mechanisms

[brenthuisman]

Yeah, elaborating a bit on the use is a good idea! I am not familiar with the build-env feature however. Would you mind typing your proposal out and adding them to this PR?

[thorstenhater]

bors try

[bors]

try

Build succeeded:

• ci/gitlab/gitlab.com

[noraabiakar]

lgtm!