Allow conditional process execution from configuration

[mahesh-panchal]

Addition:
Allow modules to use when: block by moving it to configuration.

when:
task.ext.when == null || task.ext.when

Allows modules to flexibly use the when: from the configuration file.
Allows conditional process execution on settings not available in the workflow block.

Configuration is set using process.ext.when allowing one to use process selectors and other configuration benefits to
apply a when condition to a group of processes or test process specific values.

Execution conditions

process {
    withName: 'FOO' {
        ext.args = 'module1,module2'
        // Example settings
        ext.when = null                           // Process executes - no conditional defined. 
        ext.when = true                          // Process executes - e.g. override for existing configuration
        ext.when = !params.skip_module // Process executes if set, or if null 
        ext.when = ext.args.tokenize(',').contains('module1')  // Process does not execute - evaluated in configuration context.
        ext.when = { task.ext.args.tokenize(',').contains('module1') }  // Process does not execute - evaluated in task context.
    }
}

PR checklist

• This comment contains a description of changes (with reason)
• CHANGELOG.md is updated
• If you've fixed a bug or added code that should be tested, add tests!
• Documentation in docs is updated

[mahesh-panchal]

Bonus insight from @drpatelh: Allows one to skip mirroring conditional logic in the configuration file.

[JoseEspinosa]

Really nice @mahesh-panchal , if with this we don't need to use all these ifs in the modules.config to avoid the warning regarding the process config selectors, we really owe you something 🙏

[mahesh-panchal]

I already began a trial. It's not so simple.
https://github.com/mahesh-panchal/rnaseq/tree/when_trial
It requires rewriting logic in the rnaseq workflow here to get it to work.
Just updating modules.config and all the modules brings back the WARN's. One needs to remove all the if's and change some channel assignments to mixes and so on.