PR to fix issue 90 (Pipeline fails when test.fa missing)

[travis-m-blimkie]

• Removed mentions of file "test.fa", replacing with null or empty string
• Removed schema_ignore_params argument for non-existent parameter genomes

Tested this change on real data, but recommend further review/testing before merging. Attached relevant sample sheet, command, and Nextflow log file.

nextflow_test_run_90.log
sample_sheet_test.csv
testing.sh.txt

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/bacpaq branch on the nf-core/test-datasets repository.
• Make sure your code lints (nf-core lint).
• Ensure the test suite passes (nf-test test main.nf.test -profile test,docker).
• 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).