Fix logic to accept singplex flex libraries in cellranger_multi

[delfiterradas]

PR checklist

Closes nf-core/scrnaseq#529
In reference to #9652

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[fmalmeida]

Hi @delfiterradas , many thanks for opening this PR.

@grst , I discussed yesterday with Delfina regarding this change, and I personally think it is the best.

As we keep the normal behaviour but, still give the ability to force the singleplex.

When the first edition was made regarding singleplex, I had commented something in these lines.

What do you think of it?