Add column for replicates

[JoseEspinosa]

Add an optional column in the input sample sheet for the replicates information, see #235

Closes #235

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• This comment contains a description of changes (with reason)
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[JoseEspinosa]

I drafted a possible implementation for the inclusion of the replicates column in the input spreadsheet. It is far from finished @drpatelh but I would like to know if we should go this way or if you have thought of a different approach, thanks!

[JoseEspinosa]

Closed in favour of #266