Interpret paths from blast db arguments

[FranciscoDA]

nf-core/nanoclust pull request

Many thanks for contributing to nf-core/nanoclust!

Please fill in the appropriate checklist below (delete whatever is not relevant). These are the most common things requested on pull requests (PRs).

PR checklist

• [x] 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 necessary, also make a PR on the nf-core/nanoclust branch on the nf-core/test-datasets repo
• [x] Ensure the test suite passes (nextflow run . -profile test,docker).
• [ ] Make sure your code lints (nf-core lint .).
• [ ] Documentation in docs is updated
• [ ] CHANGELOG.md is updated
• [ ] README.md is updated

Learn more about contributing: CONTRIBUTING.md

These changes allow passing an actual absolute or relative path starting with / or ./ to the --db and --tax arguments. Relative paths will be resolved against the $projectDir directory. Argument values that do not start with / or ./ will be resolved as usual.