Configs and docs

[maxulysse]

• Update CI scripts
• Add possibility to download singularity images with helper script
• Update Docs
• Improve build.nf
• Use label for processes configuration
• Update configuration files
• Disable Docker in singularity profile
• Disable Singularity in docker profile
• Disable Docker and Singularity in conda profile
• Simplify check_max() function

PR checklist

• This comment contains a description of changes (with reason)
• If you've fixed a bug or added code that should be tested, add tests!
• If necessary, also make a PR on the nf-core/sarek branch on the nf-core/test-datasets repo
• Ensure the test suite passes (nextflow run . -profile test,docker).
• Make sure your code lints (nf-core lint .).
• Documentation in docs is updated
• CHANGELOG.md is updated
• README.md is updated

Learn more about contributing: https://github.com/nf-core/sarek/tree/master/.github/CONTRIBUTING.md