Extra columns fix

[kbestak]

I've added that SPOT2CELL and MOLKARTQC can accept inputs with additional columns, as long as the first four are still x, y, z, gene (still without header and tab-separated). This was done to account for optional columns in newly created Molecular Cartography data. Skipping Mindagap with such a table was causing the pipeline to not be usable. Added the appropriate test.

Fixes: #122

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/molkart 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).

[maxulysse]

LGTM