Do not merge .fastq's per sample if there is only one .fastq

Currently, all .fastqs are joined per sample as seen here:

https://gitlab.gwdg.de/loosolab/software/scRNAseq.preprocessing/-/blob/59348712e83a4d8af4e867a304a0cfd3fff2a85f/src/fastq.snake#L15-27

However, this also happens in case there is only one .fastq per sample (thus already the sample .fastq):

data:
  WT-1:
    cDNA_reads: ['A4_L003_R2.fastq.gz']
    barcode_reads: ['A4_L003_R1.fastq.gz']
  WT-2:
    cDNA_reads: ['B4_L003_R2.fastq.gz']
    barcode_reads: ['B4_L003_R1.fastq.gz']

It takes a long time to copy the files, so it would be great if the pipeline would only would be great if the pipeline would only copy them in case there are >1 files.