write_region_file: Write temporary loci region file
In mmaitenat/ideafix: ideafix, DEAmination FIXing

write_region_file creates the file needed by samtools faidx to get the k surrounding bases of the loci defined in mutation_ids.

1	write_region_file(ids, k, tmp_filename)

`ids`	character vector containing the ids of the loci to get the surrounding bases of. Id format is CHR:POS and can be obtained by calling the function `get_mut_id` on the vcf file containing those loci.
`k`	integer with the number of bases to the right and to the left of the loci to get the genomic sequence from.
`tmp_filename`	character string naming the path to the file to write.