I am using the read_vcf function on a cluster which tabix is not add to PATH but the VCF file has been indexed and there is a .tbi file with the same name but has a .tbi suffix, but allel still warn me that tabix not found, I check the source code, I found that allel try to find tabix exec file and run it to get the .tbi file from stdout, why not just check if .tbi file is exits, if so, use it, else warn user to index the VCF manually and fall back to scanning region?