Spawn a different publishing process for each output argument.

[DriesSchaumont]

Describe your changes

Allow multiple output processes to be executed for the nextflow runner, one for each output file argument.
This allows for more flexibility in terms of configuring the publishing (e.g. defining a publishDir for each output argument separatly).

Related issue(s)

Closes #xxxx

Type of Change

• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• New functionality (non-breaking change which adds functionality)
• Major change (non-breaking change which modifies existing functionality)
• Minor change (non-breaking change which does not modify existing functionality)
• Bug fix (non-breaking change which fixes an issue)
• This change requires a documentation update

Checklist

Requirements:

• I have read the CONTRIBUTING doc.
• I have performed a self-review of my code by checking the "Changed Files" tab.
• My code follows the code style of this project.

Tests:

• I have added tests that prove my fix is effective or that my feature works.
• New and existing unit tests pass locally with my changes.

Documentation:

• Proposed changes are described in the CHANGELOG.md.
• I have updated the documentation accordingly.

Test Environment