Allow topic to be scoped to a named workflow

[racosgrove]

New feature

When a topic channel is used in a named workflow that is called multiple times via module aliasing there should be a way to scope the topic to the workflow.

Usage scenario

A named/sub workflow that is called multiple times serially via module aliasing effectively cannot use topics at present without breaking the rule of 'indirectly using a topic as input and output'. Scoped topic is highly useful for iterative workflows. The feature request is based on this discussion https://community.seqera.io/t/can-process-tasks-read-write-a-shared-path-directly-with-fusion/1523/5.

Suggest implementation

Suggestion #1

Allow topic scoping either implicitly when the Channel is defined within a named workflow block, or explicitly via a flag.

Channel.topic('my-topic', scope: 'workflow')

Suggestion #2

Keep all topic channels as global entities but allow dynamic topic naming within a process output the same way file names can be dynamic.

workflow X {
  Channel.topic("my-topic-${generation}")
  Y(generation)
}

process Y {
  input:
   val(gen)
   
  output:
    'output*.csv', topic: "my-topic-${gen}"  <- ERROR ~ No such variable: gen
}