[New Feature] Group tuple size

[Lehmann-Fabian]

Today I spent some time finding out how to group tuples when the group sizes vary. Currently, I have to do it this way.

Channel
     .from( [1,'A'], [1,'B'], [2,'C'], [3, 'B'], [1,'C'], [2, 'A'], [3, 'D'] )
     .map { key, value -> [ groupKey( key, key == 1 ? 3 : 2 ), value ] }
     .groupTuple()
     .view()

Doing it this way, requires an additional mapping, moreover it gets complex if more, and non-consecutive indexes are used. I propose to do it the following way. I implemented it as an alternative, keeping the other method valid.

Channel
     .from( [1,'A'], [1,'B'], [2,'C'], [3, 'B'], [1,'C'], [2, 'A'], [3, 'D'] )
     .groupTuple( size : { it == 1 ? 3 : 2} )
     .view()

[Lehmann-Fabian]

Hi @pditommaso, can you please review this PR.

[sonatype-lift[bot]]

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