DSL2: fix print contamination output

[scarlhoff]

Removed the meta from input and output for print_contamination module, so output of multiple samples is printed. This closes #1000. The output from print_contamination.py now prints "null" in the json and "None" in the txt output, as mentioned in #995. Downstream behaviour in MultiQC needs to be assessed in the future.

PR checklist

• [x] 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 you've added a new tool - add to the software_versions process and a regex to scrape_software_versions.py
  • [ ] If you've added a new tool - have you followed the pipeline conventions in the contribution docs
  • [ ] If necessary, also make a PR on the nf-core/eager branch on the nf-core/test-datasets repository.
• [x] Make sure your code lints (nf-core lint .).
• [x] Ensure the test suite passes (nextflow run . -profile test,docker).
• [ ] Usage Documentation in docs/usage.md is updated.
• [ ] Output Documentation in docs/output.md is updated.
• [ ] CHANGELOG.md is updated.
• [ ] README.md is updated (including new tool citations and authors/contributors).

[github-actions[bot]]

This PR is against the master branch :x:

• Do not close this PR
• Click Edit and change the base to dev
• This CI test will remain failed until you push a new commit

---

Hi @scarlhoff,

It looks like this pull-request is has been made against the scarlhoff/eager master branch. The master branch on nf-core repositories should always contain code from the latest release. Because of this, PRs to master are only allowed if they come from the scarlhoff/eager dev branch.

You do not need to close this PR, you can change the target branch to dev by clicking the "Edit" button at the top of this page. Note that even after this, the test will continue to show as failing until you push a new commit.

Thanks again for your contribution!