Feature request: Allow introspection of `workflow.output` parameters

[dvg-p4]

New feature

Allow introspecting the workflow.output.* parameters within the workflow itself

Use case

This would allow mainly allow printing info messages to the user and conditionally checking validity of output paths and modes, e.g.:

if (workflow.output.enabled) {
   if (file(workflow.outputDir).exists()) {
     if (params.clobber) {
       log.warn "Overwriting existing output" 
     } else {
       error "${workflow.outputDir} already exists, use clobber = true to overwrite"
     }
  }
  log.info "${workflow.output.mode}ing output files to ${workflow.outputDir}"
} else {
  log.info "Leaving output in nextflow workDir ${workflow.workDir}"
}

Suggested implementation

Add the config options for workflow.output to the workflow metadata object accessible within the workflow itself, as sketched out above.