Add warning when groupTuple is not complete

[adamrtalbot]

New feature

When using groupTuple, it's a good idea to use a size parameter to make sure the channel proceeds to the next step once you have all the elements. However, if the size is set too large, the channel will never be satisfied and the channel never emits the new element. This is expected behaviour, but it happens silently which can be confusing and not obvious for developers (myself and @maxulysse were stumped for a while by this).

Usage scenario

workflow one {
    Channel.fromList([
        [[id: "1"], [shard: 1]],
        [[id: "1"], [shard: 2]],
        [[id: "1"], [shard: 3]],
    ])
    | groupTuple(size: 4)
    | view
}

Output:

nextflow run .
N E X T F L O W  ~  version 23.10.0
Launching `./main.nf` [reverent_archimedes] DSL2 - revision: 55b759f45d

Suggest implementation

nextflow run .
N E X T F L O W  ~  version 23.10.0
Launching `./main.nf` [reverent_archimedes] DSL2 - revision: 55b759f45d
WARNING: groupTuple never grouped the following elements 
[[id:1], [shard:1]]
[[id:1], [shard:2]]
[[id:1], [shard:3]]