Custom gmt (11)

[WackerO]

This PR adds the possibility of providing a custom GMT file for the pathway analysis which will be used instead of doing an online query on gprofiler. Alternatively, when doing the online query, the pipeline will now attempt to download the corresponding GMT file. Closes #72

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.
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• 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).

[ggabernet]

Looks good to me, but please also add documentation about this in the usage.md docs.