prepend_labels for strelka calls runs longer than 2 hours

[shukwong]

User reported that prepend_labels for strelka calls runs longer than 2 hours
would be good to estimate the time needed from the number of samples and the number of variants (WGS/WES)

[shukwong]

also look into using bcftools annotate to replace
separate_header_and_vars, prepend_labels and recombine_headers_and_vars

[shukwong]

looking into using GNU parallel to split up the body of the VCF file to see if it helps