Reorganize CLEAN_FASTA function

[atrigila]

Reorganize CLEAN_FASTA function to the following:

if(val_fasta) {
        // Clean fasta (replace non-ATCGs with Ns, remove whitespaces from headers)
        CLEAN_FASTA ( ch_fasta )
        ch_versions      = ch_versions.mix(CLEAN_FASTA.out.versions)
        
        // Set channels: clean fasta and its index
        ch_fasta         = CLEAN_FASTA.out.output

[atrigila]

Closed via #457