Combines multiple VCF files into a single file. Input files must be sorted by their contigs
and, within contigs, by start position. Throws IllegalArgumentException if the contig lists
are not present in the input files, are not identical or if the sample lists are not the
same; this class uses the GATK to merge headers, which may throw exceptions if the headers
cannot be merged. See VCFUtils.smartMergeHeaders for details.
An index file is created for the output file by default. Using an output file name with a
".gz" extension will create gzip-compressed output.