provides a simpler way to identify pairs

handle uses case like:
12878_GAAACC_L001_R1_004.fastq.gz
12878_GAAACC_L001_R2_004.fastq.gz
12878_GAAACC_L002_R1_004.fastq.gz
12878_GAAACC_L002_R2_004.fastq.gz
12878_GAAACC_L002_R1_005.fastq.gz
12878_GAAACC_L002_R2_005.fastq.gz