scattered: [chr1, 2, /path/to/region1_chr1.vcf]
scattered: [chr1, 2, /path/to/region2_chr1.vcf]
scattered: [chr2, 3, /path/to/region1_chr2.vcf]
scattered: [chr2, 3, /path/to/region2_chr2.vcf]
scattered: [chr2, 3, /path/to/region3_chr2.vcf]
gathered: [chr1, [/path/to/region1_chr1.vcf, /path/to/region2_chr1.vcf]]
gathered: [chr2, [/path/to/region1_chr2.vcf, /path/to/region2_chr2.vcf, /path/to/region3_chr2.vcf]]