1. Moved MergedVCFLUTBase<> inside utils namespace
2. Return "" if bcf_hdr_int2id() returns nullptr in get_field_name(),
get_sample_name() in VariantHeader
3. Better comments for MergedVCFLUTBase and VariantHeaderMerger
4. Removed custom missing() functions - use gamgee::missing() only
5. Merged VCF header resides inside VariantHeaderMerger. It is
constructed using add_header(), add_headers()
functions. A const reference can be obtained by using
get_merged_header().
6. All get_*_lut() functions are now const
7. Use default destructor
8. Check to ensure all test paths for test cases for VariantHeaderMerger
(see variant_header_merger_test_path_counters)