infiles: list of input VCF file to be merge.
    outfile: name of output VCF file.
    threads: number of input files to read in parallel

def merge_vcf_files(infiles, ref_seqs, outfile, threads=1):
    '''infiles: list of input VCF file to be merge.
    outfile: name of output VCF file.
    threads: number of input files to read in parallel'''
    vars_dict = vcf_file_read.vcf_files_to_dict_of_vars(infiles, ref_seqs, threads=threads)
    _dict_of_vars_to_vcf_file(vars_dict, outfile)