Add option to exclude genes as candidate highly variable genes for integration

[KallyopeComp]

It is often preferable to exclude certain genes as candidate genes for integration. Some genes (e.g., heat shock proteins) may be highly variable but are not informative for assigning cell type.

This PR adds an option (--integration_excluded_genes) to input a list of genes (one per line) that will not be considered as candidate highly variable genes.

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!
• [N/A] If you've added a new tool - have you followed the pipeline conventions in the contribution docs
• [N/A] If necessary, also make a PR on the nf-core/scdownstream 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>).
• [N/A] Usage Documentation in docs/usage.md is updated.
• [N/A] Output Documentation in docs/output.md is updated.
• [N/A] CHANGELOG.md is updated.
• [N/A] README.md is updated (including new tool citations and authors/contributors).

[nf-core-bot]

Warning

Newer version of the nf-core template is available.

Your pipeline is using an old version of the nf-core template: 3.3.2.
Please update your pipeline to the latest version.

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