Add SILVA 138.2 for DADA2

[d4straub]

Closes #791

I tested with nextflow run d4straub/ampliseq -r add-dada2-silva-138.2 -profile cfc,test --outdir results_add-dada2-silva-138.2 -resume --dada_ref_taxonomy silva and all seemed fine.

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[d4straub]

I added the SILVA 138.2 database to sbdi_compatible_databases , could you confirm that this is fine @erikrikarddaniel ?

[erikrikarddaniel]

Sure. And if it's not it wouldn't be a big deal.