Refactor splitting up parts for cleaner code

[Joon Klaps]

Description of feature

Do like this:

ch_input_for_kaiju2krona = ch_input_classifications.kaiju
        .map{ meta, profiles -> [[meta['tool'], meta['db_name']], meta, profiles] }
        .combine( databases.map{ meta, db -> [[meta['tool'], meta['db_name']], db] }, by: 0 )
        .multiMap{
            it ->
                profiles: [it[1], it[2]]
                db: it[3]
        }

Not this:

 // run stats on all variants not only those that pass the filter
        vcf_fasta = ch_vcf.join(meta_fasta, by: [0])
        vcf       = vcf_fasta.map{ meta, vcf, fasta -> [ meta, vcf ] }
        fasta     = vcf_fasta.map{ meta, vcf, fasta -> [ meta, fasta ] }