Improve rendering of conditional blocks in DAG preview

[bentsherman]

Given the following workflow:

workflow GREET {
    take:
    message
    parallel

    main:
    if( parallel ) {
        sayHello(message).view()
    }
    else {
        println "This message is short so I'll just say it: ${message}"
    }
}

process sayHello {
    input:
    val message

    exec:
    println "This message must be pretty long if you used a process to say it: ${message}"
}

It will produce the following DAG preview:

flowchart TB
    subgraph GREET
    subgraph take
    v1["parallel"]
    v0["message"]
    end
    v2([sayHello])
    v0 --> v2
    end

Loading

The parallel input is not linked to the process because it is not linked to the process inputs, but it is an "input" in a way because it controls whether the process is run.

I think the best way to handle this would be to draw a subgraph for the conditional block (either the entire if/else or separate blocks for if and else) and connect the conditional input to the subgraph.

[bentsherman]

Similarly with this example:

workflow {
  take:
  skip_download

  main:
  if( !skip_download ) {
    ch_samples = // ...
  }
  else {
    ch_samples = Channel.empty()
  }

  emit:
  samples = ch_samples
}

The nodes in the if/else block should be wrapped in a subgraph and skip_download should be linked to that subgraph, to denote that it affects the samples output.