Utils: add a zip iterator variadic template
Merge pull request #119 from broadinstitute/mc_zip_iterator

Utils: add a zip iterator variadic template
MultipleVariantReader and MultipleVariantIterator
Merge pull request #118 from broadinstitute/jt_pqueue

MultipleVariantReader and MultipleVariantIterator
When deep copying Sam/Variant records, do not also deep copy the header

Deep copying the headers every time a record is copied is wasteful and
unnecessary, given that headers are immutable (with the single exception
of VariantHeader::advanced_merge_header(), which should be moved to a
builder)
Merge pull request #124 from broadinstitute/dr_remove_header_deep_copying

When deep copying Sam/Variant records, do not also deep copy the header
Move every boundary check to max_boundary check to avoid unnecessary checks