Fix order-dependent combine(by:) silently dropping samples in QUANT

[jonasscheid]

Unwrap the groupKey to its plain [id] map before combine(by:0) in the QUANT subworkflow. GroupKey.equals() is asymmetric, so the groupKey-vs-map combine silently dropped samples depending on task arrival order (jittery on AWS Batch). Closes #459.

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/mhcquant branch on the nf-core/test-datasets repository.
• Make sure your code lints (nf-core pipelines lint).
• Ensure the test suite passes (nextflow run . -profile test,docker --outdir <OUTDIR>).
• Check for unexpected warnings in debug mode (nextflow run . -profile debug,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).