Consider contrast files for boxplots (8)

[WackerO]

This PR closes #89, boxplots are now only generated for those contrasts listed in contrast_list/matrix if these files are provided

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 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/rnadeseq branch on the nf-core/test-datasets repository.
• Make sure your code lints (nf-core lint).
• Ensure the test suite passes (nextflow run . -profile test,docker --outdir <OUTDIR>).
• 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).

[WackerO]

I'll rerun the pipeline before and after this change and share the resulting boxplots with you @ggabernet

[ggabernet]

Looks good to me!